ESF II projekt Západočeské univerzity v Plzni

reg. č. CZ.02.2.69/0.0/0.0/18_056/0013239

Basics of Heat Treatment (TZS)

Programme + Goals

• Austenitization, IRA, ARA
• Basic methods of TZ
• TZ of structural steel
• TZ for tool steels
• Excessive stresses and strains

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Austenitization

Austenitization

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• TZ base
• Ac1 - Ac3 , Ac1 - Acm
• Conversion of perlite, then dissolution of ferrite (cementite)
• Diffusion, austenite germination and growth
• The conversion takes place in sequences
• Embryos - heterogeneous nucleation in ferrite - interface ferrite cementite
• In ferrite, due to relatedness, F-A lattices are formed as opposed to C-A
• Carbon concentrations are closer
• Number of embryos - heating, structure, composition
• Pearlitic cementite particles remain undissolved
• Then homogenization of austenite

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Austenitization

1. Perlite remains intact due to the short incubation period.
2. The conversion of ferrite to austenite begins; the adjacent cementitic particles remain undissolved. The lamellas of the cementite first reduce their thickness, then only fragments remain and at the end the globules remain.
3. Some undissolved cementite remains even though all the ferrite in the colony has transformed into austenite. The structure is austenite + cementite during this phase.
4. Cementite is completely dissolved, but austenite is still inhomogeneous in chemical composition. Higher % C is in those areas where cementite appeared. The structure is formed by inhomogeneous austenite.
5. Homogenization of austenite takes place by diffusion of carbon from the places where the cementite lamellae were originally found

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1. Perlite remains intact due to the short incubation period.
2. The conversion of ferrite to austenite begins; the adjacent cementitic particles remain undissolved. The lamellas of the cementite first reduce their thickness, then only fragments remain and finally the globules remain.
3. Some undissolved cementite remains even though all the ferrite in the colony has transformed into austenite. The structure is austenite + cementite during this phase.
4. Cementite is completely dissolved, but austenite is still inhomogeneous in chemical composition. Higher % C is in those areas where cementite appeared. The structure is formed by inhomogeneous austenite.
5. Homogenization of austenite takes place by carbon diffusion from the places where the cementite lamellae were originally found.

• Austenitization of proeutectoid steels
• The transformation of pearlitic regions above A1 begins in the same way as for eutectoid steels. The proeutectoid ferrite is austenitized rapidly as the temperature begins to rise in accordance with the binary diagram; is fully absorbed above temperature A3. Dissolution of proeutectoid ferrite is also a diffusion process. The speed is again controlled by carbon diffusion.
• In steels with approx. 0.1% C, austenite nucleates at the interface between the cementitic lamellae and the ferrite. These new austenite grains grow until they occupy the adjacent ferrite grains. Cementite dissolves during the growth of austenitic grain, but some formations still remain.

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Austenitization

• Austenitization of supereutectoid steels
• Lamellar perlite is transformed in a similar manner.
• The kinetics are a bit slower.
• The reason is that it is rougher.
• The dissolution of proeutectoid cementite is more progressive as the austenitization temperature increases from A1 to Acm. Here, too, the lamellae of cementite get thinner first.

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Austenitization

• Grain coarsening
• Coarse-grained and fine-grained steels
• Fine grain - the effect of carbides, nitrides, oxides (Al, Ti, V, Zr, Nb)

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Austenitization

• Steel overheating
• Dissolution of sulphides, nitrides and precipitation at austenitic grain boundaries
• Notch's toughness decreases
• For steels, normal purity 100 to 150°C below the combustion temperature (1,350 to 1,450)
• For steels with higher purity (400 to 450) (1,150°C)

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• Steel combustion = irreversible changes in structure (melting of boundaries, occurrence of oxides along grain boundaries)

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Austenitization

IRA, ARA

IRA, ARA

IRA, ARA

IRA, ARA

IRA, ARA

605HV

435HV

308HV

278 HV

250 HV

IRA, ARA

Prostorově centrovaná tetragonální mřížka, jedna fáze

Sferoidizovaný

IRA, ARA

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Annealing

1. Stress-reducing
2. Recrystallizing
3. Soft
4. Homogenizing
5. Standardizing

• Without recrystallization
• To stabilize the dimensions
• Long-term tempering of martensite (RA decomposition)
• 120°C, 200h or longer. Alloyed 160°C/500h (roller bed)
• Artificial aging of steels
• Acceleration of nitrogen and carbon precipitation from supersaturated solid solution (for low-carbon) (N 150, C 250)
• Annealing to reduce residual stress
• Slow heating, endurance (1 to 2h), slow cooling (30-50°C/h)
• Annealing temperature 450 to 650°C (construction)
• Lower limit = yield strength decrease
• Upper limit = tempering temperature
• The tension created by all kinds of technologies is reduced
• The tension relaxes

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Annealing

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• Soft annealing
• Above 0.4% C, homogenization. Alloys that reduce carbon diffusion or stabilize cementite = extend times. The particle size depends on the original lamellar perlite
• Martensite, bainite = fast; dependence on lamella thickness
• The mesh cement is not removed

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• Supereutectoid steels, high alloy RO, etc.
• Acceleration over Ac1
• If before that there is lamellar perlite - the only possible way
• Dependence on carbon content
• Cooling 10 to 15°C/hour
• Dependence - cooling rate over critical temperatures - fineness of carbides
• Slow cooling. All the time - stress

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• Possibility of acceleration
• Beware of large charges in the chamber furnaces
• Better continuous furnaces

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Sub-eutectodic low-alloyed, several hours endurance

Annealing

Objective: to achieve partial or complete spheroidization of perlite (carbides), reduce hardness, improve machinability (important mainly for tool steels). The goal is a ferritic matrix with globular carbides - the initial structure for heat treatment.

30 μm

30 μm

30 μm

30 μm

30 μm

Demonstration of perlite spheroidization

• Dissolution – austenitization annealing
• Heating, endurance - dissolution of phases in the matrix, cooling without precipitation
• For austenitic, stainless, heat-resistant steels
• Increases the precipitation potential
• Cooling - controlled by strengthening or suppression of precipitation (resistance to intergranular corrosion)
• in austenitic steels, carbides are precipitated along the austenitic boundaries during slow cooling of grains; the same when heated to 400 to 900°C
• Solution annealing temperature above 950°C

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Annealing

• With recrystallization
• Normalization annealing
• Rapid heating over Ac3, Acm
• Temperature compensation
• Cooling in still air
• Better for smaller products (deformation by its own weight)
• Time - opal, decarburization
• Highly alloyed steels = mixed structures (B + M)
• Removal of carbide mesh (above Acm, rapid cooling below 700°C, then slow
• Even as a final operation + removal of outer stress 500 to 600°C
• Steel castings - grain refinement + plastic properties

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Annealing

refine, uniformize, "normalize" the structure - it is performed especially after forging, where it is possible to assume diversity and unevenness of the structure

Poorly performed normalization annealing :

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• Temperature too low
• Temperature too high
• Warm-up time too short
• Too long time
• Decarburization and oxidation
• Failure to delete the original structure
• Failure to remove the welding structure

Annealing

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• Basic annealing
• Improvability of machinability, formability, decrease of hardness, stress
• Above Ac3, Acm, sometimes Ac1
• Endurance, oven cooling
• High-alloy steels + complex in shape

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• Isothermal annealing
• Improves machinability, defined by Hv
• Austenitization, over Ac3, Ac1
• Endurance - isothermal transformation
• E.g. 14109, 14209; soft annealing 15 hours, isothermal annealing approx. 40 min.
• Disadvantage - not for large products
• Necessary: rapid temperature change - two furnaces (baths), waveforms - before the isothermal cooling zone

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Annealing

• Homogenization annealing
• Balancing the chemical composition - diffusion
• 1000°C-1200°C
• 6-15 hours
• Slow cooling according to the shape
• Diffusivity of a segregating element - permissible segregation
• Oxidation, decarburization, grain growth
• Connecting with heating for forming
• Grain refinement during recrystallization between forming operations

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Annealing

Hardening

Martensitic transformation temperatures

Hardening temperatures

• Basic hardening:
• The simplest; immediately after heating
• Continuous up to ambient temperature below Ms
• In oil, in air, in water
• The difference in temperature at the surface and in the centre of the hardened object
• Risk of cracks with a large p - s difference

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Hardening

• Angled (intermittent) hardening
• Purpose: to reduce the cooling rate during the A - M conversion
• Lower stress
• Intermittent above Ms (oil - air, water - oil)
• Lower p - s temperature difference
• For complex product shapes, large tools
• Difficult temperature measurement in the first cooling environment
• We follow the cooling time in the first cooling environment

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Hardening

• Hysteresis hardening
• Purpose: reduction of residual stresses
• Higher austenitic temperatures = higher homogenization, coarsening
• Critical speed reduction = hardenability
• Big difference between the beginning and the end of cooling
• Slowly over Ar3 (Ar1), then oil
• Two furnaces

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Hardening

• Thermal hardening
• Purpose: the separation of thermal and structural stresses
• Martensitic hardening higher than critical in a hot bath above Ms
• Endurance - temperature equalization
• Without conversion to bainite
• Cooling in the air
• Salt baths

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Hardening

• Isothermal bainitic hardening
• Purpose: to obtain the required mechanical values after hardening without tempering
• Austenitization, cooling at supercritical rate. Isothermal; endurance, cooling
• Austempering
• Marquenching
• Beware of pearlitic transformation

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Hardening

• Freezing hardening
• Reduction of residual austenite
• Immediately after martensitic hardening - stabilization!
• Media - alcohol + carbon dioxide, nitrogen
• Beware of large internal stresses - austenite conversion = release

• Change in hardness vs. toughness
• Don't forget to change the dimensions
• Carbide phases have the main influence
• First stage
• For carbon steels from 100°C to 150°C
• Decomposition of high-carbon martensite to low-carbon (cubic) martensite and hexagonal carbide ε - Fe_2,4C carbide partially coherent with the initial martensite; there is a reduction in specific volume
• The structure consists of low-carbon martensite (cubic) and ε carbide, reduced stress in martensite, a slight decrease in hardness
• From the tempering curve (especially for carbon steels) it is possible to choose the tempering temperature with an accuracy of 2HRC according to the required hardness of the tool
• Second stage
• From 200 to 300°C
• Decomposition of ZA into a structure of bainitic type, at the same time further decomposition of martensite, formation of bainite slows down the decrease in specific volume and decrease in hardness
• Structure consisting of bainite, carbide ε, low-carbon martensite, slowing down the hardness drop

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Tempering

19313 (MnCr)

19437 (CrW)

• Third stage
• Between 250 and 450°C
• The final carbide - Fe₃C and ferrite is formed from low-carbon martensite. Cementite – platelet ε carbide, driving force: reduction of surface energy; recrystallization recovery, grain growth
• Structure: ferrite and cementite (sorbitol); the size of the carbide particles increases; thanks to the joining of individual cementite particles - fewer faults; hardness decreases and toughness increases
• Fourth stage
• Above 450°C
• For steels with a higher content of carbide-forming elements - Cr, W, Mo and V. For these steels, the residual austenite is only transformed when cooled from higher temperatures, see the previous slide below
• Precipitation hardening takes place (finely dispersed particles of complex carbides)
• Cementite roughening of carbon steels at lower temperatures; in the case of alloy steels, enrichment of carbide with alloying elements or formation of special carbides (above 500°C); diffusion of alloys is in progress
• The structure consists of granular perlite (globular carbide in ferrite), alloyed carbides; depending on the solubility, special carbides can be formed (secondary hardness)

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Tempering

• Tempering fragility
• In general, the toughness increases with increasing degree of tempering - the dependence may deviate in some steels
• Low temperature vs. high temperature
• Low-temperature tempering fragility
• 250°C – 400°C – line 2
• Cementite precipitation on the surface of martensite formations - during the decomposition of residual austenite
• Can be suppressed by increasing the strength of austenite grains (Al), or TMZ - the strength of grain boundaries increases, the possibility of precipitation at these boundaries is reduced by partial elimination of the phase inside the grains
• Remove by increasing the tempering temperature; spheroidization of the precipitated cementite particles occurs
• Irreversible tempering fragility - irreversible spheroidization of cementite
• It mainly concerns Cr-Mn and Cr-Ni steels
• It is less important

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Tempering

1. Ideální vztah

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• High-temperature tempering fragility
• About 550°C
• Segregation of impurities and trace elements (P, S, Sn, Bi) at the boundaries of the original austenites grains
• Ni and Cr alloys contribute to this
• These alloys increase the thermodynamic activity of the impurity in the ferrite
• Mn suppresses segregation (Mn-S)
• The process may occur
• Isothermally - line 3
• Anisothermally (with slow cooling) - occurs even at temperatures above T3, can be removed by reheating and rapid cooling - diffusion of impurities is prevented - line 4
• Further tempering and slow cooling are triggered again
• The tendency to embrittlement is limited by the addition of molybdenum or tungsten
• Avoid temperatures where isothermal brittleness occurs
• Choose accelerated cooling after tempering to higher temperatures
• Use steel with minimum impurities, alloyed with Mo and W

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1. Ideální vztah

Tempering

Influence of chromium on thermodynamic activity of carbon

Changes in the structure more dependent on temperature, see tempering parameter P = T (C + logt)

1 - Resulting tempering curve

2 - Martensite tempering

3 - Carbide precipitation

4 - Transformation of residual austenite

Tempering

Practice of TZ - carbon steels

• Low-carbon
• Up to approx. 0.25% C, cheap, malleable and weldable
• Sheets, welded structures, automatic structures, surface-treated (cementation)
• TZ - low hardenability = slight increase of Rm and Re
• Mn content in the range from 0.3 to 0.9%
• Other elements are present in small quantities, but affect the steel, then the differences between the individual low-carbon steels (from remelting)
• Sometimes the presence of boron !! In thousandths, significantly affects heat treatment
• Significant increase in carbon content
• At the same carbon content, the hardness of steels with "strip" perlite is greater than that of grained perlite. The finer the strip, the higher the hardness
• Steels with max. 0.1% C
• Usual annealing for cold-formed sheets
• Recrystallization annealing 650°C to 720°C
• Magnetic yoke annealing, switchboards 100 to 1200°C, long dwell times, the result is domain alignment
• Steels with 0.1 to 0.25 %C
• They can already provide higher hardness and strength (sufficiently hard and strong core during cementation)
• Normalization annealing is often performed (including increased machinability)

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• Czech standard 12020 (C15)
• 0.15-0.19% C, 0.6-0.9% Mn
• Relatively poor machinability compared to manganese steels
• Higher Mn content (up to 0.9%) will improve the strength after normalization or after basic annealing. At the same time, it slightly increases hardenability - it plays a role in hardening after cementation
• Normalization annealing at 925°C, cooling in air
• Basic annealing heating to 885°C, slow cooling (in the furnace)
• Water hardening
1. Directly from the cementation temperature
2. Cool to 845°C, then water
3. Reheated most commonly to 815 °C theses days
• Component production process :
• Forming
• Standardization
• Roughing
• Fine machining
• Cementation
• Hardening
• Tempering
• Sanding (maximum material removed 10% of layer thickness)

Practice of TZ - carbon steels

Masivní cementit

Normalization annealing - hardened in oil

Normalizační žíhání – kaleno na vzduchu

Practice of TZ - carbon steels

• Medium-carbon
• Carbon content 0.3 to 0.65
• Significant increase in strength and yield strength
• Steels with a content of 0.25 to 0.35
• Uses - switches, flanges, hand tools, cams, etc.
• Use in refined condition
• Higher strength than the previous group
• Surface hardening (including solid core)
• Higher internal stress
• Application - spindles, gears, shafts, axles, pinions, cams, etc.

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• Czech standard 12050 (C45)
• Normalizing annealing 900°C, cooling in air
• Basic annealing - heating to 845°C. Oven cooling to 650°C; cooling rate should not exceed 28°C/hour.
• Hardening - austenitization approx. 845°C. Can be hardened by flame, induction, nitrocementation, etc.
• Tempering - initial hardness of at least 55HRC, if properly austenitized and hardened. Tempering temperature 565°C (620 to 860MPa), 480°C (860 - 1035MPa)
• Production:
1. Normalizing annealing (not performed after rolling)
2. Basic annealing (only if necessary. Because the bars are usually delivered in a condition suitable for machining.)
3. Roughing
4. Hardening
5. Tempering
6. Fine machining

Practice of TZ - carbon steels

Rozdíly v tvrdosti během dvou měsíců po popouštění. Kaleno ve vodě, popuštěno na 475°C po dobu 1 hodiny.

• Retained austenite 20°C: 4 %
• Hardness after hardening
• Structure 99% martensite: = 58.5 HRC
• Structure 90% martensite: 90 = 55.0 HRC
• Structure 50% martensite: Hm50 = 46.0 HRC
• Minimum recommended ........... 52.0 HRC
• Hardness after normalization (perlitic structure): 185 HV
• Hardness after basic annealing (spheroidized structure): 150 HV

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• Ms = 322°C, M50 = 257°C, M90 = 112°C , M99 = -98°C
• Bs = 626°C ; B50-566°C ; B99-506°C

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C 45 ARA

Perlitic transformation

Data for T = 649°C, Start ..... 3 s; 50% ……… ..6 s

Bainitic transformation

Data for T = 466°C: Start .....: 4 s; 50% transf. in: 35 s, 99% transf. in: 5 min

C 45 - IRA

Practice of TZ - carbon steels

Hardened at 58.5 HRC

Tempering: 2 h

Practice of TZ - carbon steels

25mm

50mm

25mm

50mm

30

40

50

HRC

MPa

1000

1500

2000

• Influence of normalization annealing temperature - steel C35

Practice of TZ - carbon steels

Ochlazení C 35

Cooling rate after normalization to pearlite morphology

• Czech standard 12061 (C60)
• C60 is classified as quality steel and is equivalent to 12061. C60E is classified as stainless steel; compared to 12061, it provides higher and more extensive quality guarantees specified in EN 10083. Compared to C60E, C60R is manufactured with an increased lower limit of sulfur content for better machinability
• 0.55 to 0.65% C, 0.6 to 0.9% Mn
• It reaches a hardness of up to 65 HRC
• After hardening, the structure is martensitic, rich in carbon
• Normalization annealing 885°C, cooling in air
• Basic annealing 830°C, cooling in the furnace to 650°C max. 30°C / hour
• Hardening - heating to 815°C. Possible surface flame hardening, nitrocementation, isothermal hardening
• Isothermal hardening - typically to bainite 46-52HRC. Hardening in salt at 315°C for 1 hour. Then without tempering
• Production sequence:
1. Standardization
2. Basic annealing
3. Roughing
4. Austenitization, hardening
5. Tempering
6. Fine machining

Practice of TZ - carbon steels

• Residual austenite at 20°C: 6%
• Crack sensitivity: Relatively high
• 99% martensite: = 64.5 HRC
• 90% martensite: = 60.5 HRC
• Structure 50% martensite: 52.0 HRC
• Minimum hardness after hardening .........: 54.5 HRC
• Hardness after normalization (perlite) 210 HV
• After spheroidization 165 HV

Bainitic transformation

• Bs = 576°C ; B50 = 516°C ; B99 = 456°C

Martensite formation:

Ms = 275°C

M10 = 265°C ; M50 = 210°C

M90 = 65°C ; M99 = -145°C

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Perlitic transformation

• For T = 647°C, Start.....:4 s, 50% transf.: 9 s

Bainitic transformation

• For T = 416°C: Start .....: 6 s, 50% transf.: 60 s, 99% transf.: 8 min

Practice of TZ - carbon steels

Czech standard 12061 (C60) - ARA

Czech standard 12061 (C60) - IRA

Hardened at 64.5 HRC

Tempered: 2 h

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Practice of TZ - carbon steels

Practice of TZ - carbon steels

Surface hardening C45

Surface

Transition

Core

Practice of TZ - carbon steels

• High-carbon steels
• 0.65 to 1.5% C
• Poor formability, machinability, weldability
• Improvement = spheroidization annealing
• Steels with a carbon content of 0.65% C to 1% C used for springs
• Common processing at 56 to 60 HRC
• Tempering 200 to 250°C; upper tempering temperatures 400°C
• Low hardenability = smaller springs
• Possibility of patenting = isothermal treatment at 450 to 550°C, followed by cold drawing
• Strengths up to 2750 MPa
• Possible use: Machining knives, piston rings, hand tools

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Practice of TZ - alloy steels

• Higher strength, hardness, ductility
• Higher fatigue resistance
• They tend to coarse the austenitic grain
• Tempering resistance
• Increased corrosion resistance, wear
• Better electrical and magnetic properties

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1. Manganese steel
• Mn over 1.5%
• Slight price increase with higher hardenability, strength, hot formability
• Hardness increase at TZ
• Usual finishing
• Normalization annealing increases toughness and is common in large products
• Possible use in the formed state without TZ
• Application - rails, shafts, gears, connecting rods, screws, etc.…
• Possibility of use on some tools (steels with carbon content 0.9% and Mn 1.8%)
• Hardened in oil from 840°C, followed by tempering at 230°C
• These steels have a hard surface, higher depth of hardening, they do not have large deformations
• Hadfield steel - 1.1 to 1.4% C, 11 to 14% Mn
• When hardened in water, the structure is austenitic - ratio to carbon 10:1
• Heating to 1000°C, hardened in water (non-magnetic structure)
• Use only at temperatures up to 300°C - risk of precipitation
• Modification - hardened in water with Cr, Ni (hardening temperature 1050°C)

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Practice of TZ - alloy steels

• In Fe-C-Mn alloys, part of the manganese is dissolved in the basic metal matrix (i.e., in ferrite or austenite), part is formed by double carbide (FeMn)₃C.
• Manganese dissolved in ferrite increases its strength properties without reducing ductility.
• The S-point moves to the left, so that low-alloy steels have a purely pearlitic structure with a lower carbon content than non-alloy steels.
• As the eutectoid temperature decreases and the pearlite lamellae soften at the same time, the strength increases.
• It does not increase the resistance of hardened steels to manganese tempering.
• Manganese increases the amount of residual austenite after hardening in the higher carbon steel matrix.
• At higher carbon and manganese contents, the transformation of austenite is completely suppressed and the steels have an austenitic matrix.
• When alloying steel with manganese, it is necessary to take into account its high tendency to segregation and its low diffusion rate.
• Hadfield steel is produced for forming according to the Czech standards 17 427 17 618 or for castings according to the Czech standards 42 2920 and 42 2921.
• Furthermore, Hadfield steel with reduced manganese content and nitrogen addition is used, with a content of 1% C and 8-10% Mn and N, which has a higher yield strength than conventional steel.

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Practice of TZ - alloy steels

Practice of TZ - alloy steels

• The purpose of heat treatment is to dissolve carbides and obtain a homogeneous austenitic structure - solution annealing.
• The steel is heated to temperatures of 1030 to 1080°C, at which point the precipitated carbides turn into a solid solution.
• Persistence at this temperature; rapid cooling to prevent re-exclusion of the dissolved phase. Cooling is usually done in water. We do not have to worry about cracks, as the steel has a high enough toughness.
• This heat treatment gives a purely austenitic structure.
• Wear resistance is given by the ability to achieve significant surface reinforcement. In general, the idea that strengthening occurs is by the formation of deformation martensite.
• Fine grain steels have higher wear resistance than coarse grain steels. Hence the dwell time at the annealing temperature. On the one hand, the effort to delay is longer in order for the carbides to dissolve completely. On the other hand, the grain size matters.
• The plastic properties of crude Hadfield steel castings are always very low.
• The dissolution of the structural phases after the slow cooling of the casting is the slower during the further heat treatment, the greater the amount of accompanying structural components and phases formed in the austenite are. The dissolution of the secondary phases into austenite can be carried out in a single heating only for small castings. Larger or complicated castings must be austenitized repeatedly, while, in order to minimize stress, it is suitable to perform cooling after the first austenitization (with a longer time delay) in air and only after the second solution annealing to water perform the cooling in water.
• Heterogeneous structural conditions, especially in larger or complex castings, are prone to cracking after cooling in water. Weld crack repair is not very successful due to difficult weldability, mainly related to low thermal conductivity of manganese alloyed austenite.

Practice of TZ - alloy steels

Slow cooling = excretion of perlite at grain boundaries

Rapid cooling = austenite only

400°C/2.4 h

400°C/24 h

400°C/240 h

500°C/2.4 h

500°C/24 h

500°C/240 h

Practice of TZ - alloy steels

ii. Silicon steels

• In general, silicon in all steels (only somewhere above 0.6% = silicon steels)
• Excellent flexible behaviour, excellent electrical and magnetic properties with resistance to scaling
• Electricity steels = Si content 3 to 4%, less than 0.5% C (electric motor cores), transformer sheets
• The less carbon, the better the properties
• Necessary coarse-grained with texture
• Annealing 1100°C to 1200°C in a hydrogen atmosphere
• Steel dynamo Si content 1 to 2%
• Steels with 2% Si, 1% Mn, 0.5 to 0.7% C are often used
• High E, fatigue resistance = springs
• Hardening in oil (840 to 930°C) and tempering 400°C to 550°C

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Practice of TZ - alloy steels

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TZ of silicon-manganese spring steels

Practice of TZ - alloy steels

• iii. Chrome steels
• With carbon content of 0.1% C to 0.2% C with low chromium content of 0.5 to 1% Cr (cementation)
• The toughness decreases with increasing chromium content
• Carbon content 0.35% and 0.5% Cr
• Use for jaws of hand tools, shafts, etc.
• Usual hardening in oil/water with subsequent tempering, then excellent wear resistance with sufficient toughness
• The hardening medium depends on the geometry of the product; hardening from 870°C
• 0.5% C and 1.5% Cr - hardening from 850°C/oil; tempering to 300°C = 44HRC
• 0.9% C + 1.0% Cr - use for tools, knives, hammers, etc.; hardening 810/oil; tempering 250 to 300°C
• 0.95% C + 1.3 to 1.6% Cr = bearings; hardened 840°C/oil; tempering 150 to 160°C; hardness 62 HRC; also used for cold working tools
• The properties of these chrome steels can be increased by alloying Ni, Mo, V and W

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Practice of TZ - alloy steels

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• Czech standard 14109 (100Cr6)
• Normalization 900°C
• Soft annealing - e.g.:
• Hardening - austenitization 845°C, beware of decarburization, oil (exceptionally polymer)
• Freezing -70°C/h
• Tempering - as soon as possible - from 50°C (minimum tempering temperature 120°C) to change tetragonal martensite to cubic. The usual tempering is 150°C (it does not significantly reduce the hardness). Tempering at 175°C reduces the hardness by approx. 2 HRC. In aviation, double tempering is required. With HRC hardness requirements of 58 to 65 HRC; it is tempered to 170 to 230°C.
• Production sequence:
• Standardization
• Soft annealing
• Roughing
• Hardening
• Tempering
• Fine machining

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Different degrees of spheroidization

100%

85-90%

80 – 85%

55 – 60%

Practice of TZ - alloy steels

iv. Nickel steels

• Compared to carbon steels, higher strength, ductility, fatigue resistance, toughness, hardenability
• Generally lower hardening temperatures; high cooling intensity is not required
• Carburizing steels with increased nickel content (3.5 to 5%) = higher price, but higher ductility, strength. Use for pinions, cams, gearboxes, etc.

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74

Practice of TZ - alloy steels

• Medium carbon nickel steels (2.5 to 3.5 Ni) are generally used for machine parts
• They have optimal properties in the refined state
• Hardening from 830 to 860°C in water or oil, tempering to 550 to 650°C
• Use for aerospace parts, crankshafts, pinions, propellers, turbines, cams,
• Usually Ni is not the only alloying element in steels
• Combination = NiCrMo, NiCr, NiMo
• V. Molybdenum steels
• Significant improvement of properties - hardenability, ductility, strength even at elevated temperatures
• Molybdenum also prevents grain growth
• High price of Mo - therefore not more than 0.7% Mo
• The three most common Mo contents are 0.15 - 0.3… .0.3 - 0.45. … 0.45 - 0.7
• Low-carbon ones are for cementation: Mo ensures wear resistance and core stiffness
• Higher carbon content for highly stressed components
• TZ 10 to 20°C higher temperatures than carbon with the same carbon content
• Typical representatives are Ni-Mo, Cr-Mo are Ni-Cr-Mo.

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Practice of TZ - alloy steels

• VI. Chromium-Molybdenum steels
• Molybdenum enhances the action of chromium
• They usually contain 0.25 to 0.65% Mo
• Molybdenum increases ductility and suppresses tempering brittleness
• The use of these steels is, for example, in the aerospace and automotive industries - wear-resistant parts that are slightly strength-stressed
• Can be used for nitriding, good creep properties

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Practice of TZ - alloy steels

77

• The medium-carbon version has high strength and ductility
• Use for medium-stressed machine parts = shafts, pins, etc.
• They are suitable for induction hardening
• This type of steel with the highest carbon content can be used in the cold-formed state

Practice of TZ - alloy steels

• VII. Nickel-Molybdenum steels
• Increased hardenability, ductility, formability, suppression of tempering brittleness, etc.
• Use: the same as nickel steels
• Higher TZ temperatures generally by 10°C compared to nickel steels
• VIII. Nickel-Chromium steels
• They have a large number of advantages, minimum disadvantages
• Excellent mechanical properties, excellent corrosion resistance, high strength at elevated temperatures
• Low-carbon steels with 1.25% Ni, 0.75% Cr are cemented; this ensures wear resistance with a solid core

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Practice of TZ - alloy steels

79

• Medium-carbon steels with 1.25% Ni, 0.75% Cr; they harden to greater depths than low-carbon steels
• Used in refined condition
• Most often oil hardening plus tempering
• Crankshafts, propellers, components for the mining industry
• Steels with 3.5% Ni, 1.5% Cr have better properties - very high ductility

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Practice of TZ - alloy steels

80

Practice of TZ - alloy steels

• Nickel-chromium-molybdenum steels
• Low-carbon NCM steels are usually designed for cementation. They have a high surface hardness and core stiffness. Used for the production of gears, piston pins, shafts, drive wheels, clutches and valve rockers.
• Medium-carbon NCM steels have good strength and ductility. They have a wide range of uses expanding TZ. Typical use for shafts, bolts, screws, etc.
• Medium-carbon NCM steels with 2.5% Ni, 0.7% Cr and 0.5% Mo have high hardenability, strength and ductility even at low temperatures.

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Practice of TZ - alloy steels

• 16440 (34 CrNiMo 6); mind the difference in Mo
• High hardenability
• Mind chemical aspects and hardenability
• Normalization 870°C
• Soft annealing

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• 16440 (34 CrNiMo 6)
• Hardening temperature 845°C in oil
• Tempering: temper before cooling to room temperature
• Manufacturing process
1. Normalization annealing
2. Soft annealing
3. Roughing
4. Austenitization, hardening
5. Tempering
6. Fine machining
7. Possibly: nitriding

Practice of TZ - alloy steels

Practice of TZ - alloy steels

Data ARA Diagram

Residual austenite at 20°C: 3%

Crack susceptibility: High

• 99% martensite: 52.0 HRC
• 90% martensite: 48.5 HRC
• 50% martensite: 40.0 HRC

Minimum hardness after hardening 48.0 HRC

Hardness after normalization: 195 HV

Hardness after spheroidization: 170 HV

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Bs = 498°C; B50 = 438°C; B99 = 378°C

Martensite formation:

Ms = 326°C; M50 = 261°C; M90 = 116°C; M99 = -94°C

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Data IRA Diagram

A1 = 724°C

A3 = 771°C

Pearlitic transformation: Data for T = 650°C:

Start: 3 min

50% : 7 min

Bainitic transformation: Data for T = 338°C:

Start : 60 s

50% : 7 min

99% : 45 min

• Example - sub-hardened

16440

• REASONS FOR TOOL STEEL TZ
• Achieving the optimal combination of strength and toughness for a given application
• Achieving specific properties for a given application
• Hardening: achieving a martensitic structure in the entire cross-section of the body, or in its part, or only on the surface or functional surfaces; achieving the required tool strength and, if necessary. other features
• Maintaining the appropriate amount of carbides
• Tempering: reduction of stresses caused by hardening, achieving the final combination of useful properties (hardness, strength, toughness)

TZ for tool steels

• Tool steel classification
• Carbon-based - The hardenability of these steels is lower; it can be increased by the content of manganese, silicon or higher addition of chromium than is common with these steels.
• Alloyed for cold work
• The alloying additives are the most carbide-forming elements, Cr, W, Mo and V, as well as Ni, Mn and Si.
• Hardness depends mainly on the carbon content. At carbon contents above 1%, the hardness of hardened steels does not increase; it even decreases due to the increasing content of residual austenite.
• Alloying additives only increase hardness if they form carbides with carbon
• Chromium has a positive effect on wear resistance, increases hardenability and tempering resistance.
• Tungsten and molybdenum improve wear resistance, tempering resistance and hardenability.
• Molybdenum steels are tougher than tungsten steels.
• Vanadium most significantly increases abrasion resistance and prevents coarse grain when overheated. Nickel improves hardenability and toughness, and therefore impact resistance.
• Silicon increases the yield strength, fatigue resistance and tempering resistance.
• Chromium steels are the most numerous group, which is divided into sub-eutectoid steels with graded carbon content, chromium ledeburitic steels and chromium subledeburitic steels. The most efficient steels are ledeburitic steels. They have special chromium carbides in the structure, or vanadium, which provide excellent wear resistance. Chrome ledeburitic steels have high hardenability and are hardened in real used dimensions in the whole cross-section. Modified steels with increased toughness (reduction of carbon content) or increased abrasion resistance (addition of tungsten, molybdenum and vanadium) are derived from this type. Manganese steels have greater hardenability compared to carbon steels. They are characterized by very small dimensional changes after hardening.

TZ of tool steels

• This group includes mainly medium- and high-alloy steels. Their properties are mainly due to the carbide-forming elements Cr, Mo, W and V. These steels generally have lower carbon contents in the range of 0.2 to 0.6% C.
• All hot-rolled alloy steels have a high degree of hardenability. They are tempered at high temperatures in the area of secondary curing, where high values of strength (hardness) are achieved. For the necessary reduction of residual austenite, of which there is an increased amount in the structure, especially during hardening in air or in a warm bath, we perform repeated tempering.
• Tungsten steels (content 4-9% W) are high-performance steels with high strength and satisfactory toughness. They have good hardenability. Difficult solubility of tungsten carbides in austenite requires higher hardening temperatures. A characteristic feature is a significant maximum of secondary hardness and thus high resistance to tempering.
• Molybdenum or tungsten-molybdenum steels differ in the level of alloying. They are among the most powerful steels for hot work. A specific problem is the even distribution of the carbide phase.
• Chromium-molybdenum steels - relatively high chromium content ensures high hardenability of steels; they do not have high tempering resistance. They are tougher than previous steels.
• Nickel steels are mostly low- and medium-alloy die steels. These steels have high hardenability, and are characterized by high toughness and resistance to sudden changes in temperature. The possibility of secondary hardening decreases and, in some steels, it is completely lost.

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TZ of tool steels

• Key parameters
• Heating method - according to the type of steel (low thermal conductivity) = slow
• Placement in the furnace
• Protection against decarburization
• Salt
• Protective atmosphere
• Protective coatings
• Vacuum furnaces
• Heating rate
• Remember that during heating the strength decreases = deformation by its own weight. Using supports is necessary. Thin-walled tools – vertical use
• For warm-up times, particular equipment and experience must be taken into account

TZ of tool steels

• The austenitization temperature affects:
• Austenite structure (homogeneity, etc.)
• Coarsening of austenitic grain
• Dissolution of carbides and impurities as embryos of transformation
• Chemical composition
• Ti, V and Nb
• A large number of embryos
• Moving the perlite nose to the left
• Worse hardenability
• Better annealing conditions
• Austenitic grain size
• Better hardenability
• Easier formation of bainite during annealing
• The problem of the origin of the Wudmanstätten structure

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TZ for tool steels– austenitization

TZ for tool steels

• The austenitization temperature affects :
• Austenite structure (homogeneity, etc.)
• Coarsening of austenitic grain
• Dissolution of carbides and impurities as embryos of transformation
• Chemical composition
• Ti, V and Nb

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• A large number of embryos
• Moving the perlite nose to the left
• Worse hardenability
• Better annealing conditions
• Austenitic grain size
• Better hardenability
• The problem of the occurrence of the Widmanstätten structure

Austenitization

Preheating usually below A1

TZ for tool steels

Carbon-based possible even without preheating

The first preheating under Ac1

Second preheating above Ac3, Acm

Austenitization

TZ for tool steels

1280°C/3min., 829 HV 60, G6 , Nital 3%/18s

1180°C/3min., 795 HV 60, G8 , Nital 3%/18s

1280°C/9min., 811 HV 60, G7, Nital 3%/18s

1180°C/3min., 795 HV 60, G11, Nital 3%/18s

19436, 880°C/30 min/water

1100°C/30 min/water

1200°C/30 min/air

TZ for tool steels

• Technological process of production
• Standardization (870°C, endurance from 15 minutes for small products to 1 hour for large products)
• Roughing
• Stress-relieving annealing (ideally). 650°C to 675°C. Endurance 1h per inch of thickness. Then cooling in air.
• Fine machining
• Preheating (slow heating to 650°C)
• Austenitization (790 to 815°C) 10 to 30 minutes.
• Hardening (oil) results in about 63 to 65 HRC
• Stabilization (if possible); increases hardness and dimensional stability. At 150°C to 160°C, hold for 20 to 30 minutes, then deep freeze to -100 to -195°C. Then immediate tempering.
• Tempering. Immediately. Usually from 175°C to 260°C. Then the achieved hardness is 62 to 57HRC.
• Grinding to size
• High dimensional stability of steel during TZ. Relatively lower hardenability. High resistance to decarburization. Crack resistance at TZ.

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TZ for tool steels– 90MnCrV8 (19312)

TZ for tool steels– 90MnCrV8 (19312)

Hardening temperature x hardness

ARA

Dimensional changes with tempering temperature

Hardenability belt

IRA

Tempering diagram

• X155CrVMo12.1, SVERKER 21
• 1.5% C, 12% Cr, 1% Mo
• High hardenability, small distortions, safe, does not soften, medium resistance to decarburization, possibility of nitriding
• Soft annealing - mind decarburization. Heating to 850°C, slow cooling (10°C/hour) to 650°C followed by air. HBW approx. 210.
• Stress-relieving annealing 675°C-705°C, endurance 1 hour per inch of thickness, air. Some use 650°C/2 hours/air.
• Austenitization, hardening - very slow heating, preheating 650-750°C, austenitization 980°C-1050°C - endurance 15 min for small tools,45 min for large tools. Hardening/air, salt bath (540°C - thermal hardening), oil.
• Stabilization - ideally 150-160°C before freezing to -85°C.
• Tempering after reaching the temperature of 50-60°C. Tempering to 205-540°C
• Production sequence:
1. Roughing
2. Annealing to reduce stress (optimum)
3. Fine machining
4. Preheating, austenitization, hardening
5. Stabilization (optimal)
6. Tempering, double
7. Grinding to size

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TZ for tool steels– AISI D2, (19573)

Dependence of hardness on austenitization temperature

IRA

Dimension change during tempering

tempering diagram

TZ for tool steels– AISI D2, (19573)

Dimensional changes after hardening80x80x80mm

• Technological process of production
• Roughing
• Stress-relieving annealing (ideally). 675°C-705°C. Endurance 1h per inch of thickness. Then cooling in air.
• Fine machining
• Preheating (very slow heating to 815°C)
• Austenitization (925-980°C) 15 minutes for small tools, 45 minutes for large tools
• Hardening (in warm oil) results in about 64 to 66 HRC.
• Stabilization (ideal); it increases hardness and dimensional stability (decrease in RA - especially if higher austenitization temperatures are used). Recommended especially for complex shapes (at 150-160°C as soon as possible, then freeze to -85°C). Then immediate tempering.
• Tempering (double). Immediately. Usually from 205°C to 540°C. Cooling to room temperature between temperings. Achieved hardness of 61-54 HRC.
• Grinding to size
• Oil-hardened steel. Relatively acceptable dimensional changes. Softening resistance at elevated temperatures. Medium decarburization resistance.

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TZ for tool steels– 1.2080, D3, (19436)

TZ for tool steels– 1.2080, D3, (19436)

960°C hardened in oil

1060°C hardened in oil

hardened tempered

hardened tempered

TZ for tool steels– 1.2080, D3, (19436)

Bending strength values according to hardening method

TZ for tool steels– 1.2080, D3, (19436)

TZ for tool steels– 1.2343, H11, (19552)

Hardness vs. hardening temperature

Diagram of steel with later vs. early carbide precipitation

TZ for tool steels– 1.2343, H11, (19552)

Tempering diagram

Tempering temperature x KV

Strength at elevated temperatures

Austenitization vs. tempering temperatures

Tempering parameter

• Technological process of production
• Roughing
• Stress-relieving annealing (ideally). 650-675°C. Endurance 1h per inch of thickness. Then cooling in air.
• Fine machining
• Preheating (very slow heating to 815°C)
• Austenitization. Decarburization protection (salt, wrap, controlled atmosphere, vacuum). For preheating, it is possible to place the products in the oven at 260°C, in a wrap in the oven at 370-540°C. Heat slowly to 815°C. Heating speed max. 110°C/hour. Endurance 1h per inch of cross section. If two preheats, then the first to 540-650°C, the second to 845-870°C. Austenitization at 995-1025°C for 15 to 40 minutes. Hardened in air and sometimes in oil.
• Hardened in air and sometimes in oil, sometimes angled hardening oil/air. Salt bath 595-650°C (thermal hardening). Hardnesses from 53 to 55HRC.
• Stabilization (ideal). Increases hardness and dimensional stability (decrease in RA - especially if higher austenitization temperatures are used). Recommended especially for complex shapes (at 150-160°C as soon as possible, then freezing at -100 to -195°C). Then immediate tempering
• Tempering (double). As soon as the tool temperature is 50°C. Usually from 540°C to 650°C. Cooling to room temperature between temperings. The second tempering is necessary, a third tempering is ideal. Achieved hardness 38 to 54 HRC.
• Grinding to size
• Today, a very popular and used steel (good price-performance ratio) for hot work. Very good hardenability. Resistance to cracking during temperature cycling. Can be cemented or nitrided. High resistance to hot softening. The hardness does not change up to 590°C. It does not have significant secondary hardness. The hardness drops significantly when tempering above 565°C. High ductility, small distortions at TZ and medium abrasion resistance. Medium to excellent machinability. Medium decarburization resistance.

TZ for tool steels– 1.2343, H11, (19552)