1. HEAT TREATMENT

2. An operation or combination of operations involving heating and cooling of a metal/alloy in solid state to obtain desirable conditions (e.g., that of relieved stresses) or properties (e.g., better machinability, improved ductility, homogeneous structure)
Heat treatment is a stage in fabrication of structures.
Purpose of heat treatment
Cause relief of internal stresses developed during cold working, welding, casting, forging, etc.
Harden and strengthen metals
Improve machinability
Change grain size
Soften metals for further working as in wire drawing or cold working
Improve ductility and toughness
Increase heat, wear and corrosion resistance of materials
Improve electrical and magnetic properties

3. Fundamental principles of Heat treatment
Steel heat treatments are made possible by eutectoid reaction in iron carbon system
All basic heat treatment processes for steel involve the transformation or decomposition of austenite
Based on the principle that an alloy experiences change in structure when heated above a certain temperature and it undergoes again change in structure when cooled to room temperature
Cooling rate is important factor, slow cooling rate above critical range in steel produces pearlite whereas rapid cooling will give rise to martensite.
Steps of heat treatment
Heating a metal/alloy to definite temperature
Holding at that temperature for a sufficient period to allow necessary changes (austenisation) to occur
Cooling at a rate necessary to obtain desired properties associated with changes in the nature from size and distribution of micro-constituents

7. Thermal processing of metals
Annealing: Heat to Tanneal, then cool slowly.

8. Annealing Processes
All the structural changes obtained by hardening and tempering may be eliminated by annealing.
to relieve stresses
to increase softness, ductility, and toughness
to produce a specific microstructure
Process consists of
heating to the desired temperature
holding
cooling to room temperature
annealing time must be long enough to allow for any necessary transformation reactions

9. Normalizing - used to refine the grains
cooling in air, less expensive, some sections of a part may cool too fast
Full anneal: Utilized in low and medium carbon steels that will be machined or plastically deformed
cooling in furnace to room temperature
final product is coarse perlite (soft and ductile)
Spheroidizing
for medium and high carbon steels
Fe3C will turn into the spheroids

11. Some parts should be hard on the surface but soft and ductile inside
shafts, gears, guideways of machine tools
carious surface hardening processes
heating on the surface only and
quenching it
flame hardening (by torch)
induction hardening
carburizing

12. Process Annealing
The steel is heated to a temperature below or close to the lower critical temperature ( ), held at this temperature for some time and then cooled slowly. The purpose is to relive stress in a cold worked carbon steel with less than 0.3%wt c.

13. Stress relief annealing
Recrystallization annealing
Recrystallization annealing process consists of heating a steel component below A1 temperature i.e. at temperature between 6250C and 6750C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multi-step cold working operations. In certain case, recrystallization annealing may also be applied as final heat treatment.
It relieves stresses produced by casting, quenching, machining, cold working, welding, etc. Applies equally to ferrous and non-ferrous metals Stress relief is often desirable when a casting is liable to change dimensions to a harmful degree during machining

14. Spherodizing
Spherodite forms when carbon steel is heated to approximately 700 for over 30 hours. The purpose is to soften higher carbon steel and allow more formability. This is the softest and most ductile form of steel. Here cementite is present.

15. NORMALISING
The process of normalizing consist of heating the metal to a temperature of 30 to 50 c above the upper critical temperature for hypo-eutectoid steels and by the same temperature above the lower critical temperature for hyper-eutectoid steel. It is held at this temperature for a considerable time and then quenched in suitable cooling medium. The purpose of normalizing is to refine grain structure, improve machinibility and improve tensile strength, to remove strain and to remove dislocation.

16. NORMALISING
Normalizing is a type of heat treatment applicable to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling.
The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, you should normalize it first to ensure the maximum desired results.
Usually, low-carbon steels do not require normalizing; however, if these steels are normalized,
no harmful effects result. Castings are usually annealed, rather than normalized; however, some castings require the normalizing treatment.
Normalized steels are harder and stronger than annealed steels. In the normalized condition, steel is much tougher than in any other structural condition.
Parts subjected to impact and those that require maximum toughness with resistance to external stress are usually normalized.
In normalizing, the mass of metal has an influence on the cooling rate and on the resulting structure.
Thin pieces cool faster and are harder after normalizing than thick ones. In annealing (furnace cooling), the hardness of the two are about the same.

20. Hardening
Different techniques to improve the hardness of the steels are conventional hardening, martempering and austempering.
Conventional hardening
Conventional hardening process consists of four steps.
The first step involves heating the steel to above A3 temperature for hypoeutectoid steels and above A1 temperature for hypereutectoid steels by 500C.
The second step involves holding the steel components for sufficient socking time for homogeneous austenization.
The third step involves cooling of hot steel components at a rate just exceeding the critical cooling rate of the steel to room temperature or below room temperature.
The final step involves the tempering of the martensite to achieve the desired hardness. In this conventional hardening process, the austenite transforms to martensite. This martensite structure improves the hardness.

21. Martempering (marquenching)
This process follows interrupted quenching operation. In other words, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the center to cool to the temperature as the surface.
Further cooling is continued through the martensite region, followed by the usual tempering.
In this process, the transformation of austenite to martensite takes place at the same time throughout the structure of the metal part.

23. Austempering
Here the quench is interrupted at a higher temperature than for martempering to allow the metal at the center of the part to reach the same temperature as the surface.
By maintaining that temperature, both the center and surface are allowed to transform to bainite and are then cooled to room temperature. Austempering causes less distortion and cracking than that in the case of martempering and avoids the tempering operation.
Austempering also improves the impact toughness and the ductility of the metal than that in the case of martempering and conventional hardening.

25. Hardening 0.6% carbon steel
The metal is heated to over 723 degrees, which allows the carbon to dissolve into the FCC Austenite.
Quenching the metal quickly in water prevents the structure from changing back into BCC.
A different structure, Body Centre Tectragonal (BCT) is formed. It is called Martensite and is extremely hard and brittle with a needle-like microstructure.

26. Tempering
The hardened steel is not readily suitable for engineering applications. It possesses following three drawbacks.
Martensite obtained after hardening is extremely brittle and will result in failure of engineering components by cracking.
Formation of martensite from austenite by quenching produces high internal stresses in the hardened steel.
Structures obtained after hardening consists of martensite and retained austenite. Both these phases are metastable and will change to stable phases with time which subsequently results in change in dimensions and properties of the steel in service.

27. Tempering helps in reduce these problems
Tempering helps in reduce these problems. Tempering is achieved by heating hardened steel to a temperature below A1, which is in the range of 1000C to 6800C, hold the component at this temperature for a soaking period of 1 to 2 hours (can be increases up to 4 hours for large sections and alloy steels), and subsequently cooling back to room temperature.
The tempering temperature is decided based on the type of steel. Highly alloyed tool steels are tempered in the range of 5000C C. a good combination of strength and ductility. Low alloy construction steels are tempered above 4000C to get
Spring steels are tempered between 3000C-4000C to get the desired. It is observed that the increase in the tempering temperature decreases the hardness and internal stresses while increases the toughness.

33. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling.

37. ©2003 Brooks/Cole, a division of Thomson Learning, Inc
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

38. The CCT diagram for a low-alloy, 0.2% C steel.

39. Hardenability of Steels
Hardenability: A measure of the ability a specific alloy to be hardened by forming martensite as a result of given heat treatment
The Jominy end-quench test:
to measure hardenability
Ability to form martensite
Jominy end quench test to measure hardenability

41. The end-quench, or Jominy, test:
It fulfills the cooling rate requirements of hardenability testing most conven­iently. The test specimen, a 1-in. (25.4 mm) dia. bar 4 in. (102 mm) in length, is water quenched on one end face.
The bar from which the specimen is made must be normalized before the test spec­imen is machined.
The test involves heating the test specimen to the proper austenitizing temperature and then transferring it to a quenching fixture so designed that the specimen is held ver­tically 12.7 mm above an opening through which a column of water may be directed against the bottom face of the specimen.
While the bottom end is being quenched by the column of water, the opposite end is cooling slowly in air, and intermediate positions along the specimen are cooling at intermediate rates.
After the specimen has been quenched, parallel flats 1800 apart are ground in. (0.38 mm) deep on the cylindrical surface.
Rockwell C hard­ness is measured at intervals of 1/16 in. (1.59 mm) for alloy steels and 1/32 in. (0.79 mm) for carbon steels, starting from the water-quenched end.
Details of the standard test method are contained in spec­ifications of the American Society for Testing and Materials (ASTM Method A255) and the Society of Automotive Engineers (Standard J406); in these specifications, dimensions are given in inches.

43. Effect of Alloying Elements on Hardenability
(1.0 Cr & 0.20 Mo)
(0.55 Ni, 0.50 Cr, & 0.20 Mo)
(1.85 Ni, 0.80 Cr, & 0.25Mo)
(0.85 Cr)
(plain carbon steel)
Distance from quench end
Hardenability Curves for Five Steel Alloys (Each Containing 0.4 wt% C)

44. Quenching Mediums (1): Water
The most commonly used quenching medium
Inexpensive and convenient to use
Provide very rapid cooling
Especially used for low-carbon steel, which requires a very rapid change in temperature in order to obtain good hardness and strength
Can cause internal stresses, distortion, or cracking

45. Quenching Mediums (2): Oil
More gentle than water
Used for more critical parts, such as parts that have thin sections or sharp edges
Razor blades, springs, and knife blades
Does not produce steel that is as hard or strong as steel quenched by water
Less chance of producing internal stresses, distortion, or cracking
More effective when oil is heated slightly above room temperature to 100°F or 150°F (40°C or 65°C): reduced viscosity

46. Quenching Mediums (3): Air
More gentle than oil
Does not produce steel that is as hard or strong as steel quenched by water or oil
Less chance of producing internal stresses, distortion, or cracking
Generally used only on steels that have a very high alloy content
Special alloys (such as Cr and Mo) are selected because they are known to cause materials to harden even though a slower quenching method is used
The heated sample is placed on a screen. Cool air is blown at high speed from below it.

47. Effect of Quenching Medium
air
oil
water
Severity of Quench
small
moderate
large
Hardness
The severity of quench: water > oil > air

48. Basic concept is to heat the surface to austenitic range, then quench it to form surface martensite - work piece is steel

49. Surface Treatment
Basic concept is to heat the surface to austenitic range, then quench it to form surface martensite - work piece is steel.
The surface treatment processes are used to improve the properties of the surface only.
In many applications, it is necessary to harden the surface to prevent abrasive wear.
Different types of hardening methods such as quenching, induction hardening, carburizing, nitriding, physical vapour deposition (PVD), chemical vapour deposition (CVD) are some of the commonly used surface treatment method.
These processes are sometimes referred to as post-processing.
They play a very important role in the appearance, function and life of the product. Broadly, these are processes that affect either a thin layer on the surface of the part itself, or add a thin layer on top of the surface of the part.
There are different coating and surface treatments processes, with different applications, uses, etc.
The important uses include:
• Improving the hardness
• Improving the wear resistance
• Controlling friction, Reduction of adhesion, improving the lubrication, etc.
• Improving corrosion resistance

50. Case Hardening
Case hardening produces a hard, wear-resistant surface or case over a strong, tough core.
The principal forms of case hardening are carburizing, cyaniding, and nitriding. Only ferrous metals are case-hardened.
The steels best suited for case hardening are the low-carbon and low-alloy series.
When high-carbon steels are case-hardened, the hardness penetrates the core and causes brittleness.

51. CARBURIZING
Carburizing is a case-hardening process by which carbon is added to the surface of low-carbon steel.
This results in a carburized steel that has a high-carbon surface and a low-carbon interior.
When the carburized steel is heat-treated, the case becomes hardened and the core remains soft and tough.
Two methods are used for carburizing steel. One method consists of heating the steel in a furnace containing a carbon monoxide atmosphere.
The other method has the steel placed in a container packed with charcoal or some other carbon-rich material and then heated in a furnace.
To cool the parts, you can leave the container in the furnace to cool or remove it and let it air cool.
In both cases, the parts become annealed during the slow cooling. The depth of the carbon penetration depends on the length of the soaking period.
With today’s methods, carburizing is almost exclusively done by gas atmospheres.

52. TYPES OF CARBURIZING PROCESS:-
Gas carburizing
Liquid carburizing
Pack carburizing
Vacuum carburizing
Plasma(ion) carburizing
Salt bath carburizing

53. GAS CARBURIZING
Gas carburizing has become the most popular method of carburizing in the last two decades.
The main carburizing agent in this process is any carbonaceous gas such
as methane, propane or natural gas.
In this process it is necessary that the hydrocarbon gases should be diluted with a carrier gas to avoid heavy soot formation.
Carrier gas can be made by controlled combustion of hydrocarbon gas.
Methane can be burnt in air to methane ratio 2.5 and reacts as:
2CH4+O2 2CO+2H2
And the common endothermic carrier gas has the composition (vol. %)
N2=39.8%; CO=20.7%; H2=38.7%; CH4=0.8%
The important chemical reaction occurring during gas carburizing is:
CH4+Fe Fe(C) +2H2………. (1)
2CO+Fe Fe(C) +CO2………. (2)
CO+H2+FeFe(C) +H2O……… (3)

54. CHARCOAL
WITH BARIUM CARBONATE AS ENERGISER (10 to 15%). Process depends on presence of CO
2C + O CO
At surface, releases C atoms
2CO CO2 + C
C dissolved interstitially at surface of steel.
Ba CO Ba O + CO2
CO2 + C 2CO

55. LIQUID CARBURIZING
Liquid carburizing is a method of case hardening steel by placing it in a bath of molten cyanide so that carbon will diffuse from the bath in to the metal and produce a case comparable to the one resulting from pack or gas carburizing.
Liquid carburizing may be distinguished from cyaniding by the character and composition of the case produced. The cyanide case is higher in nitrogen and lower in carbon the reverse is true of liquid carburized cases.
Low temperature salt baths (lights case) usually contain a cyanide content of 20 percent and operate between 1550 °F and 1650° F.
High temperature salt baths (deep case) usually have cyanide content of 10 percent and operate between 1650°F and 1750° F.

56. Pack carburizing
Pack carburizing is a process of packing parts in a high carbon medium such as carbon powder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 ºC (1652 ºF).
CO gas is produced at this temperature which is a strong reducing agent. Due to high temperature, carbon is diffused into the surface as the reduction reaction occurs on the surface of the steel.
Based on experimental and theoretical calculations on diffusion theory the parts are removed and can be subject to the normal hardening methods when enough carbon is absorbed inside the part.
During the process the part which needed to be carburized is packed in a steel container and surrounded by granules of charcoal.
The charcoal is treated with an activating chemical such as Barium Carbonate (BaBO3) that promotes the formation of Carbon Dioxide (CO2). CO2 will then react with the excess carbon in the charcoal to produce carbon monoxide (CO).
Next, carbon monoxide will react with low carbon steel surface to form atomic carbon which diffuses into the steel.
Carbon gradient supplied by Carbon Monoxide is necessary for diffusion. It is to be noted that, carburizing process does not harden the steel but it just only increases the carbon content to some predetermined depth below the surface to a sufficient level to allow subsequent quench hardening.

58. CYANIDING
This process is a type of case hardening that is fast and efficient.
Preheated steel is dipped into a heated cyanide bath and allowed to soak.
Upon removal, it is quenched and then rinsed to remove any residual cyanide.
This process produces a thin, hard shell that is harder than the one produced by carburizing and can be completed in 20 to 30 minutes vice several hours.
The major drawback is that cyanide salts are a deadly poison.

59. NITRIDING
This case-hardening method produces the hardest surface of any of the hardening processes.
It differs from the other methods in that the individual parts have been heat-treated and tempered before nitriding.
The parts are then heated in a furnace that has an ammonia gas atmosphere.
No quenching is required so there is no worry about warping or other types of distortion.
This process is used to case harden items, such as gears, cylinder sleeves, camshafts and other engine parts, that need to be wear resistant and operate in high-heat areas.

66. Induction hardening
Here, the steel part is placed inside copper induction coils and heated by high-frequency alternating current and then quenched.
Depending on the frequency, the rate of heating as well as the depth of heating can be controlled.
Suitable for: Medium carbon steels (wt.% C ≥ 0.4), cast irons Hardening temperature:
The induced current i within the steel then produces heat according to the relationship: Heat = i2R,
where R is the electrical resistance of the steel.
Surface hardness achieved: 50 to 60 HRC Case Depth: 0.7 to 6 mm