Heat treatment of steels- I

This presentation gives a brief introduction to heat treatment of steels.
Keywords: Iron-carbon diagram, TTT curves, Annealing, Normalizing, Hardening, Tempering, Subzero treatment, Patenting

1. Heat Treatment Of Steels By: Nishant Khatod Assistant Professor STC, Latur

2. Cooling curve of pure iron Cooling curve of pure iron [Source: V.D. Kodgire , S.V. Kodgire , 2010]

3. Structures BCC Structure FCC Structure

4. Iron-carbon diagram iron carbon phase diagram.mp4 I-C diagram [Source: V.D. Kodgire , S.V. Kodgire , 2010]

5. Phases existing in iron-carbon diagram α -ferrite: Interstitial solid solution of carbon in BCC α -iron, at low temperatures Solubility of carbon in α -iron at room temperature is 0.008% and increases with increase in temperature to about 0.025% at 727ᵒ C Soft and ductile phase ϒ (Austenite): Interstitial solid solution of carbon in FCC ϒ -iron Phase is stable above 727ᵒC Solubility of carbon in ϒ -iron at 1147ᵒ C is 2% Soft, ductile, malleable and non-magnetic phase δ-ferrite: Interstitial solid solution of carbon in BCC δ-iron, at high temperatures Similar to α -ferrite except occurrence at high temperatures Cementite : Intermetallic compound with a fixed composition of 6.67% Orthorhomic structure Extremely hard and brittle phase

6. Three invariant reactions in I-C diagram Peritectic transformation: General reaction: S 1 + L S 2 [At constant temperature] In I-C diagram δ (0. 1 % C) + L(0.55%C) ϒ (0.18%C) [At 1492ᵒ C] Eutectoid transformation: General reaction: S 1 S 2 + S 3 [At constant temperature] In I-C diagram ϒ(0. 8% C) α (0. 025%C) + Fe 3 C (6.67%C) [At 727ᵒ C] Eutectoid mixture of α and Fe 3 C is called pearlite (average carbon content is 0.8%) Eutectic transformation: General reaction: L S 1 + S 2 [At constant temperature] In I-C diagram L ( 4.3 % C) ϒ( 2 % C) + Fe 3 C(6.67%) [At 1147ᵒ C] Eutectic mixture of ϒ and Fe 3 C is called ledeburite (average carbon content 4.3%)

7. Critical temperatures Sr. No. Critical Points [Symbols] Temperature [ᵒC] Significance during heating 1 A [Curie temperature of cementite ] 210 Cementite becomes paramagnetic 2 A 1 [Lower critical temperature ] 727 Pearlite starts transforming to austenite 3 A 2 [Curie temperature of ferrite] 768 Ferrite becomes paramagnetic 4 A 3 [Upper critical temperature for hypoeutectoid steels ] 727-910 Completion of ferrite to austenite transformation 5 A cm [Upper critical temperature for hypereutectoid steels ] 727-1147 Completion of cementite to austenite 6 A 4 1400-1492 Completion of austenite to δ -ferrite transformation

8. Termination of I-C diagram @ 5% Beyond 5% C, Carbon sublimes at atmospheric pressures instead of melting and alloys are very difficult to prepare Cementite decomposes into long and thick graphite flakes as soon as it forms and detoriates the mechanical properties

9. Transformation products of austenite Transformation of austenite to pearlite : Growth of pearlite colony in austenite [Source: V.D. Kodgoire , S.V. Kodgire , 2010] Nucleation and growth of pearlite colonies [Source: V.D. Kodgoire , S.V. Kodgire , 2010] Mic rostructures of coarse and fine pearlite [Source: R. N. Ghosh , 2006]

10. Transformation products of austenite Transformation of austenite to pearlite :

11. Transformation products of austenite Transformation of austenite to bainite : Obtained on rapid cooling below 550ᵒC Bainite is a extremely fine mixture of ferrite and cementitte Transformation starts with nucleation of ferrite Lower temperatures and hence diffusion rate is very low Upper bainite : Formed at higher temperatures and has feathery appearance Lower bainite : Formed at lower temperature and has acicular(needle) like appearance Finer distribution of carbides in lower banite than in upper bainite and hence lower bainite is stronger, harder and tougher than upper bainite Properties depend upon temperature and carbon content Microstructures of lower and upper bainite [Source: R. N. Ghosh , 2006]

12. Transformation products of austenite

13. Transformation products of austenite Transformation of austenite to martensite : Diffusionless transformation Transformation takes place by shear mechanism Instability of austenite at lower temperatures and as there can be no diffusion austenite tries to stabilize by changing its microstructure Thus FCC gets transformed to BCT Martensite is very hard strong and brittle Properties depend on carbon content Ms: Martensite tranformation starts Mf: Martensite transformation completes Ms and Mf depend upon amount of carbon and alloying element Difference between Ms and Mf is in the range of 150-210ᵒ C 99% austenite transforms to martensite , 1% remains untransformed and is called as retained austenite Microstructure of martensite [Source: R. N. Ghosh , 2006]

14. Transformation products of austenite

15. Retained austenite 99% austenite transforms to martensite , 1% remains untransformed and is called as retained austenite Austenite is relatively soft phase and its presence detracts from the hardness usually desired in a steel requiring full hardening Martensite has a BCT structure whereas austenite has FCC structure Due to this, components are like to distort or crack due to the volume changes resulting in increase in residual stresses Thus retained austenite is not a useful phase for applications like precision gauges and measuring instruments Retained austenite can be eliminated by two methods namely, subzero treatment and tempering

16. TTT diagrams [I-T diagrams] TTT diagram [Source: R. N. Ghosh , 2006]

17. Critical cooling rate It is a rate which just bypasses the nose of the TTT diagram OR Rate of cooling necessary to just suppress the diffusion transformation Critical cooling rate depends upon amount of carbon and alloying elements With increase in carbon content and alloying elements (except Co) the critical cooling rate decreases i.e. shift of nose of TTT curve to the right Shift of nose to the right gives an idea about hardenability ; lower the cooling rate higher is the hardenability Lower cooling rate reduces the tendency of warping and cracking Critical cooling rate [Source: V.D. Kodgoire , S.V. Kodgire , 2010]

18. Heat treatment Every heat treatment process consists of three steps; Heating: Small grains will combine and form large grains Heating media: Air – Non-uniform and Slow Oil – Uniform and rapid heating – Used upto 200ᵒC Salt bath - Uniform and rapid heating – Used above 200ᵒC to avoid oxidation and decarburization Holding: All grain will turn into uniform shape and size All carbides dissolve to form austenite Time of soaking depends upon Size of component Type of steel Initial microstructure Cooling: Based on rate of cooling, final grain size of the component (properties) will be decided

19. Quenching media Quenching medium should be in such that it should extract heat rapidly at high temperatures and slowly at lower temperatures Cooling media in decreasing order of cooling rates: Brine (Cold water + 5-10% salt): Reduces distortions and eliminates weak spots Costlier than water, corossive , service life is reduced Commonly used salts are NaCl , CaCl 2 , NaOH etc Cold water: Low cost, abundantly available and easy handling Used for carbon steels, alloy steels and non-ferrous alloys Bubbles can be formed which lead to soft/weak spots Water + soluble oil: Non-uniform hardness, distortion and cracks Oil: Minerals oils are used Reduces risk of distortion More suitable for alloys steels than plain carbon steels Fused salt (Salt containing large number of ions): Used for HSS Advantages over brine solutions: Uniformity in temperature, uniform heat transfer and no danger of oxidation and decarburisation Air: It should be dry Lowest cooling rate, no cracking

20. Objectives of heat treatment Increase hardness, wear and abrasion resistance and cutting ability of steels Re-softening the hardened steel To adjust mechanical, physical or chemical properties like hardness, tensile strength, ductility, electrical and magnetic properties, microstructure or corrosion resistance Eliminate internal residual stress Induce controlled residual stresses Stabilize the steel Refine grain size Increase machinability Eliminate gases like hydrogen which embrittles steel Change composition of surface by diffusion of C, N, Si etc so as to increase wear resistance, fatigue life or corrosion resistance

21. Conventional annealing Applicable: Steels with %C = 0 to 2% Objectives: Relieve residual stresses induced during cold working To soften the hardened steel To refine grain size To increase ductility To make steel suitable for subsequent heat treatment Process: Heating: Hypo-eutectoid steel: A 3 + 50ᵒ C; Hyper-eutectoid steel: A1 + 50ᵒ C Holding: Holding at this temperature for definite period (for equalization of temperature and complete austenitization ) Cooling: Furnace cooling to room temperature

22. Conventional annealing Hypereutectoid steels are always annealed from above A1 temperature and never annealed from above Acm temperature; Dislocations get blocked which induces brittleness in the steel Oxidation and decarburisation Grain coarsening Schematic representation of blocking of dislocation by cementite region [Source: V.D. Kodgoire , S.V. Kodgire , 2010]

23. Isothermal annealing Applicable: Medium carbon (0.3-0.6%C), high carbon (0.6-2%C) and some of the alloy steels Objective: To obtain improved machinability Process: Heating: Hypo-eutectoid steel: A3 + 50ᵒ C; Hyper-eutectoid steel: A1 + 50ᵒ C Holding: Holding at this temperature for definite period Cooling: Slightly fast cooling than conventional annealing to a constant temperature just below A1, holding at this temperature for completion of transformation and then cooling to room temperature in air Advantages: Reduced annealing time than conventionally annealing specially for alloy steels Homogeneity in structure Improved machinability (because of spheroids of cementite ), surface finish and less warping during subsequent heat treatment Disadvantages: Large size components cannot be subjected to this treatment Isothermal annealing (a) Hypereutectoid steel (b) hypo-eutectoid steel [Source: T. V. Rajan , C. P. Sharma, 2013]

24. Diffusion annealing

25. Diffusion annealing Also known as homogenizing annealing Applicable: High carbon (0.6-2%C) and high alloy steels Objectives: Remove structural non-uniformity Process: Heating: Above Acm (say 1000-1200ᵒ C) Holding: 10-20hrs Cooling: Air cooling (Slow cooling) Heating to such high temperatures leads to grain coarsening and hence it is always followed by another heat treatment or plastic deformation for plastic deformation Hypoeutectoid and eutectoid steel casting are given annealing and hypereutectoid steels are normalised or partially annealed High temperatures, holding times and slow cooling rates of second heat treatment makes the process highly expensive

26. Spherodise annealing Applicable: Steels with % C = 0.6-1.2 and alloy steels Objectives: Improve machinability and ductility Microstructure shows globules of cementite particles in the matrix of ferrite Methods of producing spherodised structures: Hardening and high temperature tempering Holding for long time below A1 temperature Thermal cycling around A1 A very coarse structure is to be preheated to make it fine before spherodising , to ease spherodising Typical heat treatment cycle to produce spherodised structures [Source: V.D. Kodgire , S.V. Kodgire , 2010]

27. Partial annealing Also called as intercritical annealing or incomplete annealing Applicable: Low and medium carbon steels Objectives: Improve machinabilty Process: Heating: Hypoeutectoid steel: Between A1 and A3; Hypereutectoid steel: Between A1 and Acm Holding: Definite period Cooling: Slow cooling Resultant microstructure consists of fine pearlite and cementite Less expensive than full annealing because of low temperatures involved

28. Subcritical annealing Applicable: Steels with % C less than 0.4 Objectives: Relieve internal stresses Reduce hardness Modify grain size Types: Stress-relieving: Process: Heating: 500-550ᵒ C [ Recrystallisation temperature = 600ᵒ C for steels with % C less than 0.4] Holding: 1-2hrs Cooling: Air cooling Internal stresses are partly releived without loss of strength and hardness i.e. without change in microstructure Reduces risk of distortion in machining and increases corrosion resistance

29. Subcritical annealing Recrystallization annealing: Process: Heating: 625-675ᵒ C [Above recrystallisation temperature] Holding: Definite period Cooling: Air cooling Ferrite recrystallizes and cementite tries to spherodises Internal stresses are relieved and steel becomes soft and ductile Process annealing (Intermediate annealing): Process: Same as recrystallization annealing except that it is an intermediate process Used to soften the metals during mechanical processing so as to continue the cold working process without cracking

30. Normalising Applicable: Steels with % C = 0-2% Objectives: To eliminate coarse grain structure obtained during forging, rolliing and stamping Improve machinability of hypoeutectoid steel To reduce internal stresses To increase homogeneity of structure Process: Heating: Hypoeutectoid steels: A3 + 50ᵒ C; Hypereutectoid steels: Acm + 50ᵒ C Holding: Definite period Cooling: Air cooling [ Slightly faster than furnace cooling]

31. Annealing Vs Normalising

32. Conventional hardening Applicable: Steels with % C = 0.1-2 Objectives: To harden the steel to the maximum level by austenite to martensite transformation To increase wear resistance and cutting ability of tools Process: Heating: Hypoeutectoid steels: A3 + 50ᵒ C; Hypereutectoid steels: A1+ 50ᵒ C Holding: Definite period Cooling: With a rate just exceeding critical cooling rate of that particular steel Tempering

33. Conventional hardening Hypoeutectoid steels are always hardened above A3 and not between A1 and A3 Hypereutectoid steels are always hardened between A1 and Acm Hypereutectoid steels are never hardened from above Acm

34. Selection of quenching medium for hardening Critical cooling rate must be exceeded Critical cooling rate depends more on alloying elements and less upon carbon content; Alloy steels: Less cooling rate- Air cooling High carbon steel: Slightly critical cooling rate – Oil quenching Medium carbon steel: More critical cooling rate than high carbon steel – Water or Brine

35. Hardening Methods Timed quench (Interrupted quench) [Source: V.D. Kodgoire , S.V. Kodgire , 2010] Martempering [Source: V.D. Kodgire , S.V. Kodgire , 2010]

36. Hardening methods Ausforming [Source: V.D. Kodgire , S.V. Kodgire , 2010] Austempering [Source: V.D. Kodgire , S.V. Kodgire , 2010]

37. Factors affecting hardening methods Properties of steel developed by hardening depend on various factors like; Chemical composition of steel Size and shape of the steel part Hardening cycle i.e. heating rate, hardening temperature, holding time and cooling rate Homogeneity and grain size of austenite Quenching media Surface conditions

38. Defects in hardening and remedies Low hardness and strength after hardening: Causes:- Low hardening temperatures, Insufficient soaking time, delayed quenching, slow cooling rates, retained austenite Remedies:- Subzero treatment, No delay in tempering, Selection of proper cooling rates Soft spots: Causes:- Localized decarburisation , Inhomogenous microstructure, Dirt, Improper handling during quenching, Large components leading to non-uniform heating Remedies:- Spray quenching Oxidation and decarburisation : Causes:- High temperatures heating in a furnace open to atmosphere Remedies:- Salt bath, removing decarburized layer by machining, copper plating, ceramic coating Quench cracks: Causes:- Non-uniform cooling Remedies:- High temperature tempering, Adding alloying elements to lower cooling rates, Avoid sharp changes in the part, Immediate tempering

39. Defects in hardening and remedies Distortion and warping: Distortion(Shape change): Change in size and shape of the heat treatment component due to thermal and structural changes Warping (Bending): Asymmetrical distortion of component after heat treatment Causes of distortion:- Increase in volume of steel due to martensitic transformation Causes of warping:- Change in volume during heating and cooling, Non-uniform heating and cooling, Internal stresses, Lowering component into a quenching bath in inclined position Remedies:- Slow cooling in martensitic region, Heating and quenching uniformly, applying surface hardening wherever possible Warping

40. Necessity of tempering During hardening, the outer envelope of the component will undergo cooling process immediately compared to the core of the material Outer envelope converts into hard phase, due to martensite formation but the core remains soft due to retained austenite Outer envelope is in contraction due to cooling, whereas the core remains in expansion state Thus residual stresses will be produced on the component Severe intensity of crack will be formed on the surface Hardened component possess some residual stresses and hence it should not be used directly in any application without removing residual stresses Hence tempering is to be performed to remove the residual stresses

41. Tempering Applicable: Steels with % C = 0 to 0.4% Objectives: Relieve internal stresses produced during hardening Reduce hardness Increase ductility and toughness Stabilize structure Eliminate retained austenite Process: Heating: Hardened component – 100 to 700ᵒ C Holding: 1-2hrs Cooling: Air cooling Types of tempering: Low temperature tempering [100-200ᵒ C] Medium temperature tempering [200-500ᵒ C] High temperature tempering [500-700ᵒ C]

42. Types of tempering Low temperature tempering [100-200ᵒ C]: At 200ᵒC, Martensite Low carbon martensite + ϵ carbide Structure appears dark with common etching reagents like nital and picral due separation of ϵ carbide No appreciable change in retained austenite Brittleness reduces because of decrease in internal stresses Excellent wear resistance Applications: Cutting and measuring tools Medium temperature tempering [200-500ᵒ C]: Retained austenite may get converted to bainite or decompose to form ϵ carbide and martensite . Freshly formed martensite and ϵ carbide also decompose to cementite and ferrite Microstructure consists of fine cementite and this microstructure with fine cementite is called as troosite Hardness decreases with increase in tempering temperature Toughness and ductility increases Produces maximum elastic properties Applications: coil and leaf springs High temperature tempering [500-700ᵒ C] No change in microstructure except coarsening of cementite particles Coarsening results in slight decrease in hardness and toughness Applications: Connecting rods, shafts and gears

43. Effect of alloying elements on tempering For alloy steels containing W, Cr, Mo, V etc hardness rises during tempering (secondary hardening) This is due to the separation of very hard complex alloy carbides from martensite Such steels retain their hardness upto 600ᵒ C in contrast to plain carbon steels which soften badly above 300ᵒ C High resistance to tempering makes alloy steels suitable for use above 600ᵒ C

44. Temper colors Tempering can be judged by temper colors which appear on the bright red surface and experienced eyes are guided by these colors while heating materials for tempering Colors are formed because of formation of oxide layer film, which is a function of oxide film thickness Oxide film thickness depends upon tempering temperature and holding time As the thickness increases, color changes from light straw to grey

45. Temper colors Temperature (ᵒC) Temper colors 220 Straw yellow 240 Light brown 270 Brown 285 Purple 295 Dark blue 310 Light blue 325 Grey 350 Grey purple 375 Grey blue 400 Dull grey

46. Temper embrittlement Alloy steels containing Ni, Mn , Cr, P, Sb , Sn and Ar when cooled slowly or hold for a prolonged period within a specified temperature range between 350-500ᵒ C become brittle and show marked decrease in toughness This phenomenon is known as temper brittleness or temper embrittlement Higher the cooling rate lower is the degree of embrittlement Thus, it can be eliminated by quenching in oil or water Addition of Mo, Ti, Zr also eliminates embrittlement

47. Subzero heat treatment Employed for high carbon and high alloy steels used for making tools, measuring gauges, bearings and components requiring high impact and fatigue strength coupled with dimensional stability Objective: To eliminate retained austenite Process: Cooling hardened steels to a subzero temperature, which is lower than Mf temperature. Mf temperature for most of the steels lies between -30ᵒC and -70ᵒC Stresses are developed and hence should be tempered immediately Subzero treatment should be performed immediately after hardening treatment Cooling media: Mechanical refrigeration units, dry ice, liquid nitrogen

48. Patenting Applicable: Steels with %C = 0.3-0.6 Process: Heating: Above austenitization temperature Holding: Definite period Cooling: Molten salt bath, slightly above or below nose of TTT curve Holding: Till completion of transformation Cooling: Air or water spray Microstructure: Varies from pearlite to upper bainite depending upon transformation temperature Applications: Wires, ropes and springs Good toughness to resist severe stresses encountered during wire drawing operation (90% reduction in c/s can be achieved intermediate annealing0 Patenting [Source: V.D. Kodgire , S.V. Kodgire , 2010]

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