Essentials of Physical Metallurgy for Boiler Industry - Part 3/4 - An Introduction to Alloy Steels & Stainless Steels

CHAPTER 5. Cr-Mo FERRITIC STEELS & Cr-Ni AUSTENITIC STEELS

5.1 Alloy Steels

            The so-called plain carbon steel is apt to be fancy to the extent of containing measurable quantities of at least half a dozen other elements like Mn,Si, etc. each of which has its own individual effects on the properties of the steel. But when alloying elements are added on a specific purpose and in such a quantity that the properties of the steel are significantly altered, then the steel is called alloy steel.

5.2 Ferritic and Austenitic Steels

            Carbon steels exhibit poor creep properties and are, therefore, not normally used at metal temperatures beyond 450°C. Addition of Mo in ½-1% increases the resistance of the steel to deformation at elevated temperatures due to the formation of its carbides. But these carbides are not stable at  elevated temperatures for long time.

            Addition of Cr in 1-2¼% to Mo steels stabilizes these carbides. Cr-Mo steels in various compositions like 1Cr-½Mo, 1¼Cr-½Mo and 2¼Cr-1Mo are found to exhibit good creep properties. These steels are used over a wide range of service metal temperatures i.e., from 450-575°C.

            When Cr is added in excess of 11%, it increases the resistance of the steel to corrosion. Addition of Ni depresses the LCT of the steel from 723°C to lower temperatures – 8% Ni ensures that both LCT & UCT are depressed to temperatures lower than room temperature so that the steel is fully austenitic at room temperature. 18%Cr-8%Ni steels are therefore known as austenitic stainless steels, which also exhibit good creep properties at temperatures as high as 600°C.

 

5.3 Cr-Mo Ferritic Low Alloy Steels

5.3.1 Modification of Fe-C Diagram

            Addition of Cr in steel pushes LCT up and has little effect on UCT. Mo has no significant effect upon critical temperatures. ANSI B31.1 suggests approximate LCT values for Cr-Mo steels - 745°C for 1Cr-1/2Mo steel, 775°C for 1-1/4Cr-1/2Mo steel and 805°C for 2-1/4Cr-1Mo steel. Great care is to be exercised to avoid exceeding these limits when sub-critical heat treatments like tempering are given to these steels. Considering the increased LCT and lower C content (0.05-0.15%) it is safer to give super-critical heat treatments like normalizing at a temperature at least 900°C.

5.3.2 Effect on T-T-T Diagram

            Both Mo & Cr shift the T-T-T curve towards right, which means that less cooling rate is sufficient to produce harder microstructural constituents like martensite and bainite as compared to carbon steels. In fact, simple air cooling could be sufficient to increase the hardness of Cr-Mo steels. Therefore, these steels are known as air-hardening steels.

            The amount of air-hardening of Cr-Mo steels depends on the composition and austenitizing temperatures. As the alloying percentage of Cr and Mo increases, air-hardening effect also increases. Due to this phenomenon all Cr-Mo steels have to be necessarily tempered to reduce their hardness after normalizing.

            Microstructural changes during tempering of Cr-Mo steels are similar to those in the case of carbon steels up to 500°C, and hence hardness of Cr-Mo steels steadily drops until 500°C. At about 500°C Mo carbides begin to form. These carbides give hardness to the steel and there will be no further drop in hardness till the temperature reaches 600°C. This hardening due to the formation of alloy carbides between 500-600°C during tempering is known as secondary hardening. Since the diffusion of higher sized atoms of Mo is a very sluggish process, secondary hardening is delayed till 500°C. From 600°C onwards alloy carbides get coarsened and again the hardness begins to drop.

            Depending on the selection of tempering temperature between 600-775°C, a wide range of strength of Cr-Mo steels can be obtained. To have a stable microstructure during service, tempering temperature should be selected at least 50°C above the expected service metal temperature. Material specification SA213 requires a minimum tempering temperature of 650°C and a limiting hardness of 163 HB for T11 grade. However to achieve this relatively low hardness, manufacturer may have to select a tempering temperature well beyond 700°C.

5.4 Cr-Ni Austenitic Stainless Steel

5.4.1 Sensitization

            When a standard grade of austenitic stainless steel like type 304 is subjected to a temperature in the range of 475-815°C and subsequently placed in service with corrosive environment, the steel is found to be easily attacked by intergranular corrosion. This phenomenon is known as sensitization. See fig. 7.1.

            C, Cr and Ni are in solid solution in an austenite grain free of sensitization. In the sensitizing temperature range C easily moves to the grain boundary. Since Cr is a stronger carbide former than Fe, C joins with Cr along the grain boundary. Due to the precipitation of Cr carbides, percentage of Cr adjacent to the grain boundary falls below 11% resulting in the loss of corrosion resistance. The temperature at which this occurs most rapidly is close to 650°C. Below the sensitizing temperature range diffusion rates of atoms are too low to cause the formation of Cr carbide, and at temperatures beyond the sensitizing range, Cr carbide is not stable and goes into solution back.

            The formation of Cr carbide, Cr₂₃C₆  occurs by the combination of 23 atoms of Cr and 6 atoms of C. Carbon, a small atom, diffuses rapidly through austenite grain, while Cr, a much bigger atom, diffuses much more slowly and so migration of these many number of Cr atoms from interior of the grain is ruled out. Therefore, Cr is depleted from more localized regions near the grain boundary, forming an envelope of Cr-depleted region, which is susceptible to corrosion. Thus, sensitized austenitic stainless steel is always corroded intergranularly.

5.4.2 Solution Annealing

            If the steel is sensitized during hot rolling or during fabrication, it shall be given a solution anneal to dissolve the Cr carbides completely. Because all carbides should be in solution before cooling begins, and because Cr carbides dissolve very slowly, the highest practical temperature consistent with limited grain growth is to be selected. This temperature is in the vicinity of 1095°C; practically, a temperature range of 1040-1060°C is selected.

            Cooling from the annealing temperature must be rapid enough to avoid the possibility of reformation of Cr carbide precipitates in the sensitizing temperature range, but it must also be consistent with distortion limitations; whenever distortion considerations permit, water quenching is employed.

 

5.4.3 Stabilized Grades

            Sensitizing during service can be avoided by using stabilized grades, alloyed with elements having more affinity towards C than Cr. Type 321 contains Ti and type 347 contains Cb. These elements form carbides so that there will be no free C in solid solution for Cr to get precipitated by forming carbides. Thus sensitizing is totally eliminated.

5.4.4 L-grades

            Standard grades contain a C content of 0.08% maximum. By limiting this C content to 0.03%, C available for Cr is restricted for precipitation and thus, sensitizing is controlled. These are called low C grades, designated as type 304L, for example.

            Low C grades are intermediate in their tendency to precipitate Cr carbides to the stabilized and standard grades. This characteristic of limited sensitization is useful in welding, flame cutting and other hot working operations. Nevertheless, these are not satisfactory for long time service in the sensitizing temperature range, because they are not completely immune to the formation of carbides deleterious to corrosion resistance.

5.4.5 H-grades

            Austenitic stainless steels used for high temperature service were developed primarily for corrosion resistance rather than heat resistance. Since the composition of these standard grades  may not be optimum for creep and rupture resistance, further research, sponsored by ASTM-ASME, was carried out emphasizing these properties. Research indicated that the heat treatment was the most important factor for controlling creep rate. C level of 0.04% minimum was also found necessary to assure higher creep and rupture strength. These grades are designated as type 347H, for example.

            Thus, H-grades must have a C content greater than 0.04% and must be put in service in the as solution annealed condition. H-grades have been found to exhibit up to three times the service life encountered by standard grades.

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