tempering
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Tempering is a heat treatment process that reduces the brittleness of hardened steel without significantly lowering its hardness and strength. It involves heating hardened steel to a temperature below the eutectoid temperature and allowing it to cool slowly. This process reduces brittleness by allowing the formation of tempered martensite from martensite and decreasing internal stresses. Tempering may also cause some reduction in hardness. In alloy steels, tempering can result in secondary hardening due to precipitation of alloy carbides that increase hardness even as tempering progresses.
tempering
- 1. Tempering Prof. H. K. Khaira Professor MANIT, Bhopal
- 2. • Steels can be heat treated to high hardness and strength levels. The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation. • As-quenched hardened steels are so brittle that even slight impacts may cause fracture. Tempering is a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use.
- 3. Tempering • Tempering consists of heating a hardened steel to a temperature below eutectoid temperature and keeping it at that temperature for a specified time to reduce brittleness followed by air cooling. • The aim of tempering is to decrease brittleness of hardened steel.
- 4. Tempering • The steel, after hardening, contains martensite which is accicular and is very brittle. It can not be used as such. Hence it is essential to temper it to make it less brittle. • During tempering, the martensite hardness may also get reduced to some extent.
- 6. Properties of hardened steel • Hardening produces matensite which is a super-saturated solid solution of carbon in α (BCC) iron. Hence it is hard and brittle. The fig. below shows variation of hardness of martensite with carbon content.
- 8. Tempering • Tempering : a process of heating a martensitic steel at a temp. below the eutectoid temp. to make it softer and more ductile. ' reheat Fe 3C : Fe3C particles precipitates from the ’ phase → tempered martensite → spheroidite spheroidite
- 10. Tempering
- 13. Properties of hardened steel • Hardening also introduces high internal stresses making steel brittle. • Accicular nature of martensite also causes triaxial state of stress causing brittleness.
- 14. Changes During Tempering • Martensite is a metastable phase. The equilibrium phases are ferrite and cementite. • During tempering, martensite changes to low carbon martensite and then to equilibrium phases (ferrite and cementite) resulting in reduction in brittleness. • The internal stresses also get reduced making steel less brittle.
- 15. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure (a) The unit cell of BCT martensite is related to the FCC austenite unit cell. (b) As the percentage of carbon increases, more interstitial sites are filled by the carbon atoms and the tetragonal structure of the martensite becomes more pronounced. 15
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- 17. Quenching • Austenitizing : heating the steels to a high enough temperature until they convert to at least partial austenite. • Quenching: Media – brine (salt water), fresh water, oil and air • Tempering – Reheat to 200 - 550°C, decrease hardness, regain ductility (i.e. martensite). 17
- 18. Stages of tempering The overlapping changes, which occur when high carbon martensite is tempered, are divided into four stages. • • • • First stage Second stage Third stage Fourth stage or Secondary hardening
- 19. First Stage of Tempering • First stage (50-200°C) • Martensite breaks down to a low carbon martension and transition precipitate known as ε-carbide (Fe2.4C) across twins. • Second stage (205-305°C) – Decomposition of retained austenite to bainite and decrease in hardness. • Third stage (250-500°C) – Conversion of the aggregate of low carbon martensite and ε - carbide into ferrite and cementite, which gradually coarsens to give visible particles and rapid softening.
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- 21. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Effect of tempering temperature on the properties of eutectoid steel. 21
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- 23. Figure 11.29 Tempered martensite in steel ( 500). (From ASM Handbook, Vol. 9, Metallography and Microstructure (1985), ASM International Materials Park, OH 44073.) 23
- 24. Fourth Stage of Tempering • Secondary hardening ocures during fourth stage of tempering.
- 25. Secondary Hardening • In some steels, the hardness increases instead of decreasing during tempering. This is known as secondary hardening. • It ocurs due to precipitation of alloy carbides. • Steels containing carbide forming alloying elements show secondary hardening. • It ocures during fourth stage of tempering.
- 26. Secondary Hardening • Steels containing W, Cr, V etc. show secondary hardening. • High speed steel containing 18% W, 4% Cr and 1% V show secondary hardening. • High speed steels are used as tool steels. During machining, the temperature of the tool tip increases causing softening in other steels. But High Speed Steels retain their hardness for a longer time due to secondary hardening and therefore can be used for longer time.
- 27. Secondary Hardening • Secondary hardening occurs during fourth stage of tempering (400-700°C ) • Carbide changes in alloy steel at 400-700°C. In steels containing one alloying addition, cementite forms first and the alloy diffuses to it. When sufficiently enriched, the Fe3C transforms to an alloy carbide. • After further enrichment this carbide may be superseded by another and this formation of transition carbides may be repeated several times before the equilibrium carbide forms. In chromium steel, changes are: Fe3C→Cr7C3 → Cr23C6.
- 28. Secondary Hardening • In steels containing several carbide-forming elements the reactions are often more complex, and the carbides which decompose are not necessarily followed by carbides based on the same alloy elements. • The transformation can also occur in situ by gradual exchange of atoms without any appreciable hardening; or by re-solution of existing iron carbides and fresh nucleation of coherent carbide with considerable hardening that counteracts the normal softening that occurs during tempering.
- 29. Secondary Hardening • In some alloy steels, the hardness is maintained constant up to about 500°C or in some cases it rises to a peak followed by a gradual drop due to breakdown of coherence and coalescence of the carbide particles. This agehardening process is known as secondary hardening and it enhances high temperature creep properties of steel. • Chromium, for an example, seems to stabilise the size of the cementite particles over a range 200-500°C. • Vanadium and molybdenum form a fine dispersion of coherent precipitates (V4C3Mo2C) in a ferrite matrix with considerable hardening. When over-ageing starts the V4C3 grows in the grain boundaries and also forms a Widmanstätten pattern of plates within the grain
- 30. Changes in Hardness During Tempering
- 31. Changes in Hardness During Tempering
- 32. THANKS