Essentials of Physical Metallurgy for Boiler Industry - Part 2/4: Fundamentals of Heat Treatment

CHAPTER 4. HEAT TREATMENT

4.1 Recrystallization

Crystalline grains of solid steel try to reduce their surface energy to go to a more stable form. As a result, they try to reduce their outer surface. This explains their tendency to assume equiaxial shape and to grow.

These fundamental recrystallizing forces, which promote spheroidising and grain growth are present at all temperatures but the forces are greater as the crystals become finer and depart further from spherical shape. Opposed to these recrystallizing forces are strength and rigidity of the material, which will drop as the material is heated. Crystals of a material will begin to merge and grow at a particular temperature, known as recrystallization temperature.

4.2 Annealing

Also known as full annealing, it consists of heating the steel to the austenitising temperature (UCT + 20-30 °C) and soaking it there for a definite time and then cooling it back slowly to room temperature, usually in the furnace.

Let us consider a 0.25% carbon steel valve casting, conforming to the specification SA216 WCA, solidifying in the mould from liquid state. Since it passes through very high temperatures, i.e., 1400-1100ºC for sufficient time, grains get coarsened. The coarse-grained austenite at UCT is transformed into coarse-grained ferrite and pearlite at LCT. Such a coarse grained casting is brittle and weak, which can be, however, made strong and ductile by refining the grains by annealing. See fig.4.4.

The casting is reheated to its austenitising temperature. UCT of steel can be computed by assuming the UCT curve to be a straight line: 910 – [(910-723) × 0.25/0.8] ≈ 850ºC. Add 30ºC to get 880ºC as the austenitising temperature. Since diffusion rate is dependent on the surface area of a grain, coarse grains, due to their less surface area, dissolve very slowly. Therefore, austenitising temperature is selected around 900ºC. For the same reason, soaking time at this temperature is also unusually longer – 3 - 6 hours are quite common. Then the casting is slowly cooled through the transformation range between UCT and LCT in the furnace with fuel cut off. Since the microstructure of the casting does not change below LCT, casting is usually taken out of the furnace after 500°C. Thus a strong and ductile casting with fine grained structure at room temperature is obtained by annealing. Annealing also reduces the hardness of the casting.

4.3 T-T-T Diagram

            For heat treatment processes like annealing, involving slow heating and slow cooling, phase transformations follow Fe-C diagram. But for heat treatment processes like normalizing and quenching, involving slow heating but rapid cooling, Fe-C diagram is no longer useful to trace the phase transformation; instead, Time-Temperature-Transformation diagram as shown in Fig.4.2 is used.

            As cooling rate is increased, the temperature at which phase transformation takes place is lowered below LCT. As the transformation temperature gets lowered, C diffusion becomes difficult and, less ferrite and more carbide phases are formed, all leading to stronger and harder steel with less ductility.

Refer to fig.4.2. At cooling rate represented by line ‘a’, which is equivalent to equilibrium  cooling, coarse-lamellar pearlite in addition to ferrite is obtained. At a more rapid cooling rate ‘b’, transformation takes place at a temperature between 600ºC - 500ºC and fine lamellar pearlite is formed. See fig.4.3.

            When the cooling rate is such that the cooling line ‘d’, completely misses the nose of the T-T-T curve, transformation temperature gets sharply reduced to below 300ºC. At such a low temperature C cannot diffuse out of austenite to form carbides. Therefore, austenite gets supersaturated with C atoms, gets distorted and elongated. This needle shaped constituent with very high hardness and very low ductility is what is called the martensite. Some austenite grains also get retained without any transformation.

            When the cooling rate is between ‘b’ and ‘d’ e.g. ‘c’, carbides consisting of ferrite and cementite with hardness in between those of pearlite and martensite are formed. These carbide aggregates are collectively called bainites. Carbides formed at higher transformation temperatures (500 - 400ºC) are called upper bainites, which are lath-like in morphology and those formed at lower temperatures (400-300ºC) are called lower bainites, which are acicular in morphology.

            When rapidly cooled steel is reheated slowly, phase transformation takes place well below LCT, again disobeying Fe-C diagram. This is explained as follows. As the shape of the microstructure constituents, formed as a result of rapid cooling, deviates greatly from equiaxial form, recrystallizing forces will be very great. These driving forces are of sufficient magnitude to drive out C atoms at a temperature lower than LCT. As a result of this softer microstructure constituents are formed.

4.4 Quenching

Also known as  hardening, it  refers to heating the steel to its austenitising temperature, soaking it there for a definite time and then quenching it in a liquid medium.

            Let us take the case of a safety valve spring steel rod conforming to specification A322 Gr 5160 with 0.6%C - 0.8% Cr. To get maximum stiffness of the spring and to carry maximum load without plastic deformation for a given section of rod, we require as high a yield strength and hence a tensile strength as practically possible. Tensile strength of the order of 1500 MPa is obtained by quenching the spring steels.

            The above spring steel, supplied in as hot rolled condition, consists of ferrite and pearlite in the microstructure. After the spring is coiled, it is heated to its austenitising temperature of around 850ºC. It is soaked there for a time period of 1-1.25 minutes per mm. It is then suddenly quenched in oil. The quenched microstructure consists of martensite and retained austenite with maximum tensile strength and hardness. See fig.4.5.

            Martensite is formed due to the inability of diffusion of C atoms at low temperatures. Coarser grains have less surface area per unit volume, which makes diffusion more difficult. Therefore coarse-grained steels are more hardened than fine-grained steels. For an example, steels with grain size 3 can be quenched to a hardness 50% more than that to which steels with grain size 8 can be quenched. As the C content of steel increases, hardness of the quenched steel also increases. e.g. A 0.25% C-steel can be quenched to a maximum of 50 HRC whereas 0.6%C steel to above 60 HRC.

4.5 Tempering

It refers to heating the steel to a temperature below LCT, soaking it there for a definite time and then cooling it in air to room temperature. As-quenched martensite has very high strength but very low toughness and ductility. Therefore, almost all steels that are quenched to martensite are also tempered in order to improve ductility and toughness. Depending on the temperature and soaking time, tempering can produce a wide variety of microstructures and hence properties.

            As-quenched martensite microstructures are supersaturated with C, have high residual stresses, contain a high density of dislocations, have a high lath boundary area per unit volume and contain retained austenite. All these factors make martensite microstructures very unstable and drive various phase transformations and microstructural changes during tempering right from 150ºC onwards. After 200ºC, retained austenite gets transformed into ferrite and cementite. Between 250-700ºC, martensite gets transformed into progressively softer carbides. These carbides are collectively known as tempered martensite.

            Tempering of martensite between 200 - 700ºC results in progressively decreasing tensile strength, For an example, Gr 5160 0.6%C spring steel develops a hardness of HRC 60 with as quenched martensitic microstructure. After tempering at 538ºC, its hardness drops to HRC 25 with a microstructure of tempered martensite and ferrite. See fig.4.5.

            During cooling no phase transformation takes place and therefore, cooling rate is not important; usually, air cooling is adopted.

            It is very important to note that only steels with a microstructure containing martensite, bainite or fine pearlite, produced by rapid cooling, respond to tempering.

4.6 Sub-Critical Annealing

 It consists of heating the steel to a temperature below LCT, usually 25-75ºC below LCT, soaking it there for a definite time and then cooling it slowly, usually in furnace.

            Let us see the case of swaging of ends of bank tube conforming to specification SA192. Cold working like swaging and bending results in a highly strained and deformed crystal grains. A grain seen under microscope actually contains many sub-microscopic divisions. This, in turn, results in higher strength and hardness at the expense of ductility. This phenomenon is known as work-hardening or strain-hardening.

            The as-swaged bank tube may crack at the swaged end while expanding on to the drilled holes of drums. It may also crack during service because the swaged ends cannot yield to accommodate the high bending stresses due to large thermal expansion of the bank tube assembly.

            Swaged bank tube is heated to a temperature around 690ºC and soaked for a time at the rate of about 2½ minutes per mm. Since the grains are elongated and hence deviate far from the most stable equiaxed ones, recrystallization forces come into play. These forces drive the grains to assume equiaxial shape even at a temperature below LCT. Now the steel regains the original ductility, strength and hardness. After recrystallization the steel is cooled slowly back to the room temperature.

            Let us see a case study to illustrate the importance of sub-critical annealing of swaged ends of bank tubes. Entire bank tube assembly was renewed for an 18 year old 68 TPH–32 kg/sq cm industrial boiler located in the U.S. The bank tubes are SA178 Gr.A steel, 4.5mm thick and swaged to 2” from the original OD of  3¼”. Swaged portion of one of these tubes cracked during the initial start-up operation of the boiler after the renovation work.

            Visual examination of the failed tube revealed a 6” long brittle crack in the swaged section with no evidence of any wall thinning along the crack. See fig.4.6. Hardness measurements were made – swaged section averaged HRB 89 whereas the original tube, HRB 60. The microstructure of the swaged end revealed a heavily cold-worked condition with elongated ferrite and distorted pearlite colonies. A sample of the failed swaged end was heated in a lab furnace at 620°C for 15 minutes and allowed to cool in the furnace. Microstructure of this sample showed the equiaxial grains indicating the removal of the effects of cold work  - hardness of this sample now measured HRB 55.

            Since this was the only failure in several hundred tubes, it was concluded that this tube missed the heat treatment.

            In the manufacture of seamless steel tubes, cold drawn tubes are given a heat treatment of sub-critical anneal. Specification SA192 requires that cold-finished tubes shall be heat treated after the final cold finishing, at a temperature of 650ºC or higher.

4.7 Normalising

 It consists of heating the steel to its austenitising temperature, soaking it there for a definite time and then cooling it back to room temperature in air.

            Consider the hot-forming of 0.25% C-steel pipe of size OD 244.5 × 25 mm thick, conforming to specification SA106 Gr.B, into an elbow of size OD 219.1 × 22.85 mm thick, conforming to specification SA234 WPB. Forming operation is carried out in a temperature range of 1100 - 1000ºC. At such a high temperature grain growth occurs and grains get coarsened. To refine the grains so as to regain strength and ductility the as-formed fitting is given a normalizing heat treatment. See fig.4.7.

            The fitting is slowly heated to its austenizing temperature.UCT = 910-[(910-723) × 0.25/0.8] ≈ 850ºC. Add 30°C to dissolve the coarse grains to get an ausenizing temperature around 900ºC. The fitting is soaked at this temperature for a time at the rate of 1-1.25 minutes per mm. The fitting is then cooled in air to room temperature.

Specification SA234 requires that hot formed carbon steel fittings finished at temperatures in excess of 980ºC shall subsequently be annealed, normalized or normalized and tempered.

4.8 Normalising vs Annealing

 Major difference between these two heat treatments is the final strength of the heat treated steel. Normalised steels are always stronger than annealed ones. Though both steels contain ferrite and pearlite in their microstructures, pearlite is not the same in these two microstructures. During annealing, pearlite is formed at a temperature of about 700ºC. This pearlite is a mixture of alternate layers of coarse lamellae of ferrite and cementite (see Fig. 4.3), known as coarse pearlite.

            During normalising pearlite is formed at a temperature between 600ºC - 500ºC. At this low temperature C diffusion is restricted. This results in fine pearlite with more number of finer ferrite and cementite striations per unit volume of pearlite as compared to coarser pearlite. Therefore, normalised steels are stronger and harder than annealed steels.

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