Heat Treatment of Steels-I PDF

# UNIT 3 Heat Treatment of Steels-I ## PART-A ### SHORT QUESTIONS WITH SOLUTIONS **Q1. Define heat treatment. What are the stages involved in it?** **Answer:** The term "heat treatment" is a process involving controlled heating and cooling of a material in order to modify its microstructure or a...

# UNIT 3 Heat Treatment of Steels-I ## PART-A ### SHORT QUESTIONS WITH SOLUTIONS **Q1. Define heat treatment. What are the stages involved in it?** **Answer:** The term "heat treatment" is a process involving controlled heating and cooling of a material in order to modify its microstructure or alter its physical, mechanical or chemical properties. Heat treatment involves transformation or decomposition of austenite. The products obtained will determine the physical and mechanical properties. Heat treatment of steel involves, beating the material to a temperature above the critical range to form austenite. **Stages in Heat Treatment** 1. Heating: It involves steel or component to desired temperature. 2. Soaking: It consists of holding the component at that particular temperature until uniform change in the properties throughout the cross-section is obtained. It is known as holding stage. 3. Cooling: This stage involves cooling the component to room temperature at required rates. **Q2. Why is heat treatment of steels done?** **Answer:** **Reasons for Heat Treatment** 1. To produce required combination of microstructure and mechanical properties to fulfill its purpose. 2. Increases the corrosion and wear resistance, improves machinability, surface conditions and mechanical properties by refining the grain size, for relieving the internal stresses and softening the material. Various heat treatment processes used to modify the structure and properties are annealing, normalizing, hardening, tempering and surface hardening. **Q3. What is annealing?** **Answer:** Annealing is a heat treatment given to cast irons in order to soften the casting, to improve their machinability and ductility. For most gray cast irons, annealing is done at a temperature range of 850°C to 900°C to soften it and thus improve its machinability. For spheroidal graphite cast irons, annealing is done to increase its toughness and ductility. The white cast iron has cementite structure which is very hard and brittle. So, annealing heat treatment process is used to make the white cast iron soft i.e., malleable. This typical malleabilizing treatment consists of heating white cast iron at a temperature of around 900°C for long time. **Q4. What are the needs of annealing process?** **Answer:** Following are the needs or requirements of annealing. 1. Component to be hardened (material) 2. Equipment (i) Furnace (ii) Quenching tank (iii) Quenching medium (air, water, oil, brine solution etc.) (iv) Thermocouples. The required equipment are selected, based on the material of the component to be hardened and the magnitude of properties needed to be imparted within the component. **Q5. When will you prefer annealing?** **Answer:** The purpose of annealing is. 1. To refine grain size 2. To relieve internal stresses 3. To produce a definite microstructure 4. To improve ductility and reduce hardness 5. To soften the metals and improve machinability. **Q6. 'Normalized steels are stronger than annealed steels'. Explain.** **Answer:** Normalized steels are stronger than annealed steels because of the following reasons, 1. In normalizing, the steel is heated to above the upper critical temperature and then cooled in still air to room temperature. This increases the hardness and strength of the steels. 2. The increase in cooling rate effects the transformation of austenite and the resultant microstructure in many ways, 3. The increased cooling rate results in less proeutectoid ferrite in normalized hypoeutectoid steels and less procutectoid cementite in hypereutectoid steel. This leads to good strength, whereas, the proeutectoid cementite is in annealed steels resulting in less strength. 4. The faster cooling rate in normalizing will also effects the temperature of austenite transformation and the fineness of pearlite, which results in good strength and hardness. In case of normalized steels, the cementite plate in pearlite are closer, thereby stiffens the ferrite. This results in increase hardness. From the above mentioned points, it is clear that the normalized steels are stronger and harder than the annealed steels. **Q7. Differentiate between annealing and normalizing.** **Answer:** | Annealing | Normalising | |:---|:---| | Process of annealing is employed, to improve the softness and ductility of steels or metals. | Process of normalising is employed, to to refine the grain structure of steels. | | It takes longer time. | It is less time consuming process. | | Cooling is done in the furnace itself, so it is a slow process. | Cooling is done in still air. | | The quality of machined surface of annealed normalized steels is low. | The quality of machined surface of steel is high. | **Q8. What are the objectives of hardening?** **Answer:** Hardening is always followed by tempering process. Its main objectives are as follows, (i) To improve hardness for wear resistance. (ii) To enable steel strength, so that it can be used for cutting other metals.. (iii) To improve mechanical properties such as toughness, strength and ductility. (iv) To develop best combination properties of strength and notch-ductility. **Q9. Hardening should never be a final heat treatment for steel. Why?** **Answer:** Hardening should never be a final heat treatment for steel as during the process internal stresses are developed and untempered martensite is formed. This martensite is hard, but brittle in nature, resulting in the reduced toughness Thus, the ability of steel to withstand impact without undergoing failure, reduces. Hence, hardening is followed by tempering, which is done to increase the toughness of steel and to reheve the high internal stresses produced to it. During tempering, the untempered martensite transforms into iron carbide particles, which increases the strength and toughness. **Q10. What are the factors should be considered while selecting a quenching medium?** **Answer:** Factors to be considered during the selection of the quenching medium are as follows, 1. Temperature 2. Degree of agitation (mixing) 3. Type of medium used 4. Surface conditions of the component. **Q11. What is the effect of inadequate time of soaking of steel even at appropriate temperature of hardened steel?** **Answer:** Hardening of steel is done by heating it to a temperature of 30-50°C above critical temperature and holding the metal at this temperature for the phase transformation. The critical cooling is done by quenching in water, oil etc. Before quenching steel is soaked at hardening temperature in furnace to obtain stable structure of austinite. Soaking time is usually 1 hour per 25 mm thickness. Soaking time should be long enough to allow the transformation with homogeneous austenite structure. Generally, soaking time depends on the thickness and the amount of alloying elements. **Q12. What is the purpose of normalizing and hardening?** **Answer:** **Normalizing** This is one of the simplest heat treatment frequently applied to casting, forgings etc., to refine grain structure, relieve stresses and to increase strength and hardness. Normalizing is a process of heat treatment given to gray cast iron by heating it to a temperature above the transforination range i.e., 885°C to 927°C. For cast irons, with chilled layers, normalizing is done by cooling it in air from 900°C to 920°C temperature. **Hardening** Hardening is a process of heat treatment applied for cast irons by heating it to a temperature range of 860°C to 900°C. It is applied to cast iron to increase its strength, hardness and abrasion resistance. **Q13. What is cold stabilization? Why is it adopted?** **Answer:** Cold stabilization is defined as a temperature cyclic process to relieve internal stresses without affecting the temper (i.e., tougness of the material). During the process, the retairted austenite is converted into martensite at sub-atmospheric temperature in the material. Cold stabilization is adopted due to following reasons, 1. It removes the excess tartrates (salt (or) ester of organic compounds) and other materials, which causes cloudiness in the metal. 2. Less amount of energy is required for processing, which reduces the cost of manufacturing. **Q14. What is temper - embrittlement phenomenon?** **Answer:** Temper-embrittlement is a phenonenon of reduction in impact strength or toughness of steels upori tempering of quenched steels at specified temperature ranges. All commercial alloy steels are temper-embritted upto some extent. Generally. some alloy steels like Ni Cr and Mo-Crare more susceptible to this phenomenon compared to plain carbon steels. Impurities present in these steels such as P, As, Sn, Sb etc., make them more sensitive towards temper-embrittlement. However, this effect can be prevented to some extent by adding the molybdenum to the steel composition in small quantities. Usually in most of the alloy steels this phenomenon occurs at two tempering temperature zones as shown in the following schematic figure between impact strength and temperature. **Q15. Give reasons for extremely high hardness of martensite.** **Answer:** **Hardness of Martensite** Hardness of martensite is mainly due to the presence of carbon. The hardness of martensite increases with the increase in carbon content (i.e., hardness of martensite is directly proportional to carbon content). A graphical representation is shown in figure below, **Q16. Define hardenability.** **Answer:** **Hardenability** Hardenability is the measure of hardening capability of a material. It gives an idea about how easily a material can be hardened under given cooling conditions. It determines the depth and distribution of hardening in the transformation of austenized steel to martensitic steel. There are two methods to find out the hardenability of steel. They are (i) Grossman's method (ii) Jominy end quench test Out of these two tests jominy end quench test is most commonly used method. Hardenability depends on the size of grain in the austenite. Coarse grained austenite has more hardenability than fine grained austenite. **Q17. Distinguish between hardness and hardenability.** **Answer:** | Hardness | Hardenability | |:---|:---| | Hardness is defined as the ability to resist scratch, wear and tear etc. | Hardenability is the property of steel, which determines the depth and distribution of hardness obtained by quenching from an anesthetization temperature. | | Hardness is not a fundamental property of a metal but is related to the elastic and plastic properties. | Hardenability is a very important property in heat treatment of steels. | **Q18. What is the effect of cobalt addition on hardenability?** **Answer:** **Effect of Cobalt Addition on Hardenability** The addition of cobalt to steels enhances the cooling rates and speed up the pearlitic transformation. This ultimately results in decreasing the hardenability. Cobalt is one of the alloying elements of steels that has negative effect on hardenability by rapid decomposition of austenite. The effect of cobalt depends on the percentage of composition as follows, (i) When cobalt ranges from 0.2% - 0.5%, the effects such as kinetics of phase transformations are neglected but reduces the hardenability. (ii) 3% addition of cobalt reduces the retained austenite composition and also strengthens the steel. Finally, reduces the hardenability of steel. Though it has some merits, addition of cobalt is not preferred in normal steels due to its high cost. **Q19. What are the limitations on the use of isothermal transformation diagrams?** **Answer:** **Isothermal Transformation (IT) Diagram** A typical Time-Temperature Transformation (TTT) diagram used for determination of time for transformation of one phase to other phases isothermally occuring at different temperatures. **Limitations of I-T Diagram** (i) It cannot be used for direct prediction of accurate course of transformation under continuous cooling. (ii) It is used for a single group of samples only. (iii) An error may occur in the cooling rate with use of isothermal nose. (iv) Undesired transformation of austenite to bainite occurs with the use of I-T diagram. (v) It is applicable only for single isothermal temperature. ## PART-B ### ESSAY QUESTIONS WITH SOLUTIONS ### 3.1 ANNEALING, NORMALIZING, HARDENING AND TEMPERING AND SPHEROIDISING **Q20. Explain the various stages in a heat treatment cycle.** **Answer:** The process employed for controlled heating and cooling of the given material to change its microstructure and properties is termed as "heat treatment". In general, a heat treatment cycle has three stages. They are, 1. Heating 1. Heating 2. Soaking (maintaining the heat at constant rate) and 3. Cooling. **1. Heating** Steel is heated to austenite temperature at which its microstructure changes to austenite. The process used are annealing and normalizing. **2. Soaking** The temperature of austenite is held for a sufficient period to allow a uniform change in the properties throughout the cross-section and it carried out by austenizing. **3. Cooling** Steel from the austenite temperature is then cooled to room temperature. The rate of cooling depends on the desired properties associated with change in the size, form and nature. Examples are ferrite, pearlite etc. The purpose of heat treatment is to obtain a required microstructure, mechanical or physical properties of the steel being heat treated. Some of the purposes for heat treatment of steels are, (i) To relieve the internal stresses developed during cold working, casting, forging etc. (ii) Improve the hardness and strength. (iii) Improve electrical and magnetic properties. (iv) Improve ductility and toughness. (v) Soften the metals for other uses (cold working, rolling). **Q21. "Much of the versatility of steel as an engineering material arises from the fact that it is amenable to heat treatment". If so, elucidate this statement with reasons.** **Answer:** Steel is a composition of pure iron and a small percentage of carbon in its simplest form. At ordinary temperature it consists of pure iron i.e., ferrite combined with iron carbide known as cementite. The properties of the steel are affected by the composition and the structure. Therefore, heat treatment is the process by which properties of steel can be changed to obtain the desired properties. The application of steel in a particular field depends upon its properties. The variation in properties of steels apart from heat treatment can also be performed, subjecting the steel to mechanical working and alloying, but heat treatment is generally preferred. The process of heat treatment involves a combination of heating, holding and subsequent cooling at a control rate to obtain the desired conditions. The various types of the heat treatment processes applied to steel and their definite purposes are listed below. | S.No. | Heat Treatment | Purpose | |:---|:---|:---| | 1. | Annealing | To soften the steel, to relieve internal stresses. | | 2. | Normalizing | To refine the grain structure. | | 3. | Hardening | To increase hardness and wear resistance. | | 4. | Tempering | Removes brittleness from the hardened steel. | | 5. | Surface hardening | To provide wear resistance with a tough core. | Therefore, all the above stated reasons support the fact that the versatility of steel as an engineering material is because as it is amenable to heat treatment. **Q22. Differentiate between full annealing and process annealing.** **Answer:** (a) **Process Annealing** The process of low carbon steels with carbon content less than 0.3% under cold working conditions is called "process annealing" or "subcritical annealing". This process relieves strain hardness for further processes. Process annealing usually takes place below the lower critical tenmperature. This is carried out to remove the effect of cold work and to permit further cold work. A high degree of softening is induced with process annealing as it associates only with partial recrystallization of the ferrite, which is distorted. No phase change occurs in process annealing and the contents ferrite and cementite remains in the structure throughout the process. This process is carried out in batch type or in continuous furnaces in an inert atmosphere containing coal gas. (b) **Full Annealing** Full annealing is a process of heating a ferrous alloy and cooling it slowly within the furnace through the transformation range. The austenitizing temperature for different steels is given below. 1. For hypoeutectoid steels - 723°C to 910°C 2. For hypereutectoid steels - 723°C and 3. For austenizing - 1130°C. The annealing temperature depends on the amount of carbon in the steel. The steel is not heated above the upper critical temperature, because when this heated steel is cooled slowly a network of pearlite is surrounded by cementite and so the formation of relatively brittle steel is obtained. Figure (b) shows the temperature range of a heat treatment for steels performed on full annealing process. This process of full annealing is carried out to improve the softness, machinability, hardness and some other properties. **Q23. Discuss in detail about subcritical annealing.** **Answer:** The process of heating certain cold worked steels to a particular temperature less than the lower critical temperature and cooling them is called subcritical annealing. This process is mainly employed for following reasons. 1. Relieving internal stresses 2. Modifying or refining the existing microstructure 3. Reducing the hardness. The following are the most commonly used subcritical annealing processes. **1. Stress Relief Annealing** This type of annealing is mainly employed for treating the steels with low carbon content i.e., hypoeutectoid steels with carbon percentage less than 0.4%. Steels are heated to a temperature just below the recrystallization temperature (approximately 600°C) i.e., temperature ranging from 500 to 550°C. Then the steels are held at that particular temperature for a certain time i.e., about 1 to 2 hours. Later, the steels are cooled to room temperature in the atmospheric air. This particular process is mainly employed for the following reasons. (1) To relieve the internal stresses partly without affecting the hardness ad the strength. (ii) To lower the distortion while machining. (iii) Improve resistance towards corrosion. **2. Recrystallization Annealing** This process of annealing is performed on cold worked steels at a temperature ranging from 625°C to 675°C. Where, ferrite recrystallizes and cementite starts to spheroidise. This particular process is mainly employed for, (i) Relieving the internal stress along with making steels soft and ductile. (ii) Grain size refinement (depends on the degree of cold working done on the steels). **3. Process Annealing or Intermediate Annealing** It is one of the subcritical annealing processes or in-process annealing, is a heat treatment is performed by heating the steels to a temperature above the recrystallization temperature that restores some of the ductility to a material being cold-worked so it can be cold-worked further without getting cracks. This process is mainly suitable for low-carbon steel. Cold-worked steel normally tends to possess increased hardness and decreased ductility, as a result, it cannot be cold worked. Process annealing imparts softness and ductility. **Q24. What is spheroidizing annealing and normalising?** **Answer:** **Spheroidzing Annealing** Spheroidize annealing is a heat treatment process used to improve the machinability of steels. The annealed hypereutectoid steel has a combination of pearlite and cementite microstructure. The cementite present in steel gives poor machinability, because it is difficult to cut the cementite plate using the cutting tools. Also, while machining, the surface becomes irregular and uneven. Therefore, spheroidizing annealing process is employed to achieve the machinability of annealed hypereutectoid steels. In this process, spheroidal or globular form of carbide in a ferritic matrix is produced as shown in the figure below. 1. Holding the steel for a long period of time at a temperature slightly below the lower critical line. 2. Alternate heating and cooling of the workpiece component between the temperatures that are slightly below and above the lower critical line. 3. The steel is heated to a temperature slightly above the lower critical line and then it is cooled very slowly in a furnace or it is held for sometime at a temperature just below the lower critical line. **Normalizing** The process in which steels are heated to a temperature in the range of above upper critical temperature, and held at that temperature followed by cooling in still air is called normalising. Steels are subjected to normalising in order to obtain following objectives. 1. Elimination of coarse grain structure 2. Relieving internal stresses 3. Enhancement of machinability 4. Improvement of strength. During normalising process, specimens are heated to a temperature above upper critical temperature i.e., hypoeutectoid steel specimens are heated above the critical point 'A,' whereas hypereutectic steels above the critical point 'Am'. At this temperature austenite structure is formed. Then, specimens are held until temperature in the specimen is uniform throughout the structure. After holding, specimens are kept in still air and allowed to cool. Cooling rate of normalising is faster than annealing, therefore normalising results in fine grain structure comparatively. Also normalised steels has higher and lesser values of hardness and ductility respectively when compared to annealed steels. Mostly forgings and castings are normalised to obtained improved hardness or machinability. **Q25. Explain briefly, the process of normalizing.** **Answer:** The process in which steels are heated to a temperature in the range of above upper critical temperature, and held at that temperature followed by cooling in still air is called normalising. Steels are subjected to normalising in order to obtain following objectives. 1. Elimination of coarse grain structure 2. Relieving internal stresses 3. Enhancement of machinability 4. Improvement of strength. During normalising process, specimens are heated to a temperature above upper critical temperature i.e., hypoeutectoid steel specimens are heated above the critical point 'A,' whereas hypereutectic steels above the critical point 'Am'. At this temperature austenite structure is formed. Then, specimens are held until temperature in the specimen is uniform throughout the structure. After holding, specimens are kept in still air and allowed to cool. Cooling rate of normalising is faster than annealing, therefore normalising results in fine grain structure comparatively. Also normalised steeds has higher and lesser values of hardness and ductility respectively when compared to annealed steels. Mostly forgings and castings are normalised to obtained improved hardness or machinability. **Q26. Distinguish between annealing and normalizing.** | Annealing | Normalizing | |:---|:---| | Process of annealing is employed to improve the softness and ductility on steels or metals which have been worked before that. | Process of normalizing is employed to refine the grain structure of steels which has undergone cold or hot working. | | It takes longer time. | It does not take much time. | | Cooling is done in the furnace itself, so it is slow process. | Cooling is done in still air. | | The quality of machined surface of annealed steels is low. | The quality of machined surface of normalized steel is high. | | This is used to, (i) Improve machinability. (ii) Relieve internal stresses caused by a heat treatment in the previous phase. Grain structure is coarse. | This is used to, (i) Improve machinability. (ii) Release internal stresses caused by cold working in the previous phase. Grain structure is finer. | | Annealing treatment is more expensive than normalising. | Compared to annealing it is a less expensive method of heat treatment. | | Annealed steels are less harder than the normalized ones. | Normalized steels are harder than the annealed steels. | | Improves machinability of medium碳 steels. | Improves machinability of low碳 steels. | **Q27. Explain briefly hardening of steel.** **Answer:** Process which involves heating the steel above transformation range followed by soaking and quenching is called hardening. **Objectives** 1. To improve wear resistance 2. To enable steel to cut other materials 3. To enhance toughness and strength. Hardening process involves heating hypocutectoid steel to 30°C to 40°C above upper critical point i.e., A, and hypereutectoid steel is heated to same temperature range above lower critical point. Steel is soaked at that temperature in furnace until desired level of structure transformation is obtained. Then, steel is cooled by quenching process in which specimen is suddenly cooled by immersing it in water or oil. In hardening, soaking time is usually 1 hour per 25 mm thickness. Due to quenching, austenite gets transformed into fine grained martensite in case of hypoeutectoid steels, thereby steel becomes hard as structure obtained hard. In case of hypereutectoid steels, some portion of cementite do not decompose into austenite while soaking. Therefore, structure obtained upon quenching consists of both cementite and martensite. Hence, hardened hypereutectoid steels possess even more hardness and wear resistance as cementite is harder than martensite. Hardening is performed on machine components which requires high wear resistance and hardness, also on cutting tools. Wear resistance and hardness of components varies in direct proportion with cooling rate. Very high cooling rates may result in cracking and distortion of components. Hardening should never be a final heat treatment for steel, as during the process internal stresses are developed and martensite formed is hard, and brittle in nature, resulting in reduced toughness. Thus, the ability of steel to withstand impact loads without undergoing failure reduces. Due to these reasons hardened steel cannot be used directly for industrial applications. Hence, hardening is followed by tempering, which is done to increase the toughness of steel and to relieve the high internal stresses produced in it. **Q28. Discuss the characteristics of quenchants for effective removal of heat from a workplace.** **Answer:** **Characteristics of Quenchants** 1. Quenching is used for rapid cooling to obtain certain hardness and mechanical property requirements. 2. Avoids undesirable internal microstructure to ensure uniform mechanical properties, minimize residual stresses and avoid warpage. 3. It reduces crystallinity. 4. Increases toughness of both alloys and plastics. 5. Hardens the steel by introducing martensite, where the steel is rapidly cooled through its eutectoid point (the temperature at which austenite becomes unstable). 6. Extremely rapid cooling through quenching can prevent the formation of all crystal structure, resulting in amorphous metal or metallic glass. 7. Decreases the fluorescence intensity of a given substance. 8. Quenching, heavily depends on pressure and temperature, which results in excited state reactions, energy transfer, complex formation and collisional quenching. **Q29. Explain the mechanism of heat removal during quenching.** **Answer:** Consider a small steel cylinder subjected to quenching process with warm water. A typical cooling curve of cylinder with different stages of cooling is shown in figure. This curve represents the change in temperature with respect to time. Cooling rate may be defined as the rate of change of temperature with time. The cooling rate is obtained by simply drawing a tangent to the curve at that temperature on cooling curve and finding the slope of the tangent. The slope shows the variation of cooling curve. The mechanism required for removal of heat during quenching is summarized into three stages. They are, 1. Vapour blanket stage 2. Vapour transport stage 3. Liquid cooling stage. **Q30. Explain about the following heat treatment operations:** (a) Solution-heat treatment (b) Age hardening. **Answer:** (a) **Solution Heat Treatment** Solution heat treatment is one of the strengthening mechanisms used to increase the strength of the metals. It is also called solid solution strengthening or hardening. It involves a heat treatment process, where alloying constituents are added into the solution and later solidified rapidly by quenching. Strength of the metal can be indirectly defined as the resistance offered by the solute atom against the moving dislocation. Strengthening of the metal involves avoiding of this moving dislocation by means of creating a stress field around the solute metal atoms. As the atomic diameters of every element is different from each other solute atoms size will also be different from solvent atoms. When solute particle is added to the solvent particle, lattice distortion is created due to the difference in the size of atomic diameters. Larger solute atoms will cause the compressive stress field against the moving dislocation and smaller solute atoms will produce a local tensile stress field Whenever a load is apphed, this dislocation moves layer by layer of atoms and will be stopped by the stress field created by smaller and larger solute atoms. More energy will be required to overcome this resistance and thus, strength is increased. Amount of the solute particles added. Atomic size difference between solute and solvent particles and nature of distortion produced by the solute atoms are the three important factors that influence the strength produced during solution heat treatment. **(b) Age Hardening** Age hardening is one of the strengthening mechanisms, in which the strength of the metal is obtained either by keeping it at room temperature for some time or by heating it to some extent of the higher temperatures. An alloy must satisfy the below the conditions to go under age hardening. (i) With decrease in temperature, the solubility of solute in the solvent must decrease. (ii) The precipitate surrounded by the matrix should be coherent. (i.e The crystal structure of the precipitate should have a continuous relation with the matrix composition) (iii) The motion of the moving dislocation is stopped by coherent precipitate particles. The large elastic distortion of the matrix counter-attacks the stress field of moving dislocation and thus, increasing the strength. The common steps involved in the age hardening process are heating (solutionizing), quenching and aging. The alloy is heated in a furnace and held at this temperature for some time to form a single phase solid solution. This single phase solid solution is quenched further to obtain super saturated solid solution. The hardening of this super saturated solid solution is called aging  Aging is done either at room temperature (natural aging) or with a precipitation heat treatment (artificial aging). In artificial aging. the kinetics of precipitation is increased by heating it about 15% - 25% of the temperature difference between solutionizing temperature and room temperature. As overaging decreases the hardness, aging is stopped at the instance, where optimum hardness is reached. **Q31. Carburized components are subjected to subsequent hardening heat treatment. However, nitrided articles will not be heat treated subsequently? Explain why.** **Answer:** Due to prolonged heating at high temperature in the carburizing operation, both core and the case exhibit overheated structures, which are unsatisfactory for severe service Thus, the carburized parts are therefore heat treated, (a) To refine the core (b) To refine and harden the case. During carburizing process, the carbon atoms are penetrated on to the surface. Unlike carburizing, nitriding is carried out at temperatures below the stable austenitic state. The process consists of heating a component in an atmosphere of ammonia gas and hydrogen, at the temperature of 500 C to 530°C. The surface is unevenly filled by N. atoms and turns it smooth. Hence, wear is reduced and wear resistance is increased. Penetration is difficult into the iron lattice because of large atomic diameter. No phase change occurs after nitriding. The depth hardness that may be expected from a nitriding operation as compared to a carburized and hardened steel is shown in the following figure. **Q32. Why hardening of steels is followed by tempering? Discuss the different stages of tempering and draw the resulting microstructures.** **Answer:** Tempering is a heat treatment process that is carried out for hardened steels. It is immediately done after hardening and high internal stresses produced due to hardening are relieved. It increases ductility of the steel and reduces hardness. The process consists of heating hardened steel to a temperature below .f (critical temperature) and holding it for some time followed by cooling in air to room temperature, Tempering is classified into the following types depending on the transformation behaviour. (a) Low temperature tempering (100°C - 200°C) (b) Medium temperature tempering (200°C - 500°C) (c) High temperature tempering (500°C - 700°C). **Q33. Discuss the effect of carbon on tempering based on, (i) Original structure, morphology of martensite (ii) M-M, temperature and thus resultant amount of phases (iii) Hardness of martensite.** **Answer:** **(i) Original Structure, Morphology of Martensite** The process of reheating the steels to reduce brittleness with minimum loss of its hardness is known as "Tempering process". Martensite is a super saturated interstitial solid colution of carbon in a-iron. Martensite is formed, when usenite is cooled below 200°C temperature. It is a abfiusionless transformation'as no phase transformation depends upon the diflusion (since rate of diffusion is lowered) This transformation is a displaceable and every individual atom cooperatively moves without disturbing the neighbouring atoms Driving force is required to move the Motas and tou the reaction. The driving force increases as transformation temperature increases. Martensite has a body centered tetragonal structure. During tempering process, the effect of carbon on morphology of martensite and original structure are as follows, 1. Ejection of carbon from body centered tetragonal lattice of martensite 2. Isothermal transformation of retained austenite 3. Formation of ferrite carbide mixture 4. Growth and spheroidization of carbide particles. The effect of carbon content is represented in figure shown below, **(ii) M-M, Temperature and Thus Resultant Amount of Phases** M and M, means the start and end of martensite transformation respectively. M, is a definite temperature for a given steel and this temperature depends upon the chemical composition. The relationship between transformation temperatures (i.e., M) and chemical composition is given as, M(°C) 561-474 (%C) -33(% Mn) 17(% Ni)-17(% Cr) -21(% Mo) From the above equation, it is seen that, the carbon content mainly effects the M. and M, temperatures. As the carbon content increases, in composition of steel, the M, and M, temperatures will be lowered (i.e., proportion of carbon content is inversely proportional to M, and M, temperatures). By varying coolmy rates, M temperature cannot be lowered or raised. Steel is heated to austenizing temperature, which affects the M temperature and chemical composition of austenite and austentic grain size. Depending upon the priority. M. temperature increase or decrease with respect to austenizing temperature The effect of carbon content on M, and M, temperature of steel is shown in figure below. **(iii) Hardness of Martensite** Hardness of martensite is mainly due to the presence of carbon. The hardness of martensite increases with the increase in carbon content (i.e., hardness of martensite is directly proportional to carbon content). A graphical representation is shown in figure below, From the above graph, it is observed that minimum amount of carbon should be present in steels after martensite transformation, to resist minimum distortion. To obtain maximum hardness, the total amount of carbon content should be in austenite phase and cooling rate must be adequate enough to produce martensite structure. **Q34. Distinguish between annealing and tempering.** | Annealing | Tempering | |:---|:---| | Process of annealing is employed to improve the softness and ductility of steels or metals which are already worked. | Process of tempering is employed to improve ductility toughness and to reduce hardness. | | This process relieves the internal stress resulted due to cold working, welding, etc. | This relieves the internal stress induced due to quenching during hardening. | | Materials are heated to a temperatures higher than eutectoid temperature. For hypoeutectoid steels, 30° C to 50°C above A, line. For hypereutectoid steels, 30°C to 50°C above A, line. | Materials are heated to a temperatures less the eutectoid temperatures i.e., 100°C to 700°C (below A line). | | Cooled in furnance itself. | Cooled to room temperature by air cooling. | | In this process, new phases are formed. | This process increases toughness and shock resistance of metal. | | Materials acquire homogenous chemical composition. | Retained austenite is eliminated. | | Applications Steel castings, rolled products. | Applications Chisels, connecting rods, shafts and gears. | **Q35. Indicate the temperature range of the following heat treatment on Fe-Fe,C equilibrium diagram, (i) Annealing (ii) Normalizing (iii) Hardening (iv) Tempering.** **Answer:** (1) Temperature range for heat treatment processes. **Annealing** **Normalizing** **Hardening** **Tempering** **Q36. Explain in detail the different transformations (Pearlitic, baintic and martensitic) of a eutectoid steel with a suitable T-T-T diagram.** **Answer:** Transformations **1. Pearlite:** When the cooling of eutectoid steel from austenite temperature is carried out in such a way that the cooling curve passes through TTT diagram as shown in figure i.e., curve passing through region A to region P results in the transformation of austenite into pearlite. Coarse and fine pearlite is formed based on the temperatures at which transformation begins. Coarse pearlite is formed when the transformation is initiated just below the e