Steel martensitic

10 Fracture Mechanics Approach to Fatigue Crack Propagation 

Carbon steels present the same characteristic on independence of FCGR, da/dN, of the metallurgical factors in Region II of growth, but show a different general trend. This can be seen in Fig. 10.27 for type A 533 Gr B and A 508-2 steels, used in the construction of nuclear pressure vessels. 

The 280 experimental data come from different authors [43—48] and were obtained in air at RT with 0.1 R 0.2. The FCGR, da/dN, in Region II can be fitted to the line of equation 

Stainless steels can be divided into two main families austenitic and ferritic. The different structure, y for the austenite and a for the ferrite, seems to have an appreciable effect on FCGR properties of steels. This can be seen in Fig. 10.30 that collects 383 data obtained at RT by different researchers [29, 30, 49-53] on four austenitic stainless steels of the series 300, type 304, 316, 321 and 350, the last two with columbium-tantalum and titanium, respectively for high temperatures use (430-820 °C), an austenitic-ferritic (14 % S ferrite) steel type 351 cast [54, 55], a ferritic stainless steels, type 18Cr-Nb [56] and experimental data relative to 403 type martensitic stainless steel [57] (Fig. 10.28). 

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Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

An important item in this array of matenals is the class known as maraging steels. This group of high nickel martensitic steels contain so Htde carbon that they are often referred to as carbon-free iron—nickel martensites (54). Carbon-free iron—nickel martensite with certain alloying elements is relatively soft and ductile and becomes hard, strong, and tough when subjected to an aging treatment at around 480°C. 

The body-centered-cuhic (bcc) metals and alloys are normally classified as undesirable for low temperature construction. This class includes Fe, the martensitic steels (low carbon and the 400-series stainless steels). Mo, and Nb. If not brittle at room temperature, these materials exhibit a ductile-to-brittle transition at low temperatures. Cold working of some steels, in particular, can induce the austenite-to-martensite transition. 

Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, fresh water, and organic exposures). 

Barth, C. F., Steigerwaid, E. A. and Troiano, A. R., Hydrogen Permeability and Delayed Failure of Polarised Martensitic Steels , Corrosion, 25, 353 (1969)... 

For further details of the special martensitic steels. References 1 to 4 should be consulted. 

Fig. 3.12 Strength (U.T.S.) of two 13% Cr, martensitic steels as affected by tempering at various temperatures for 1 h following air cooling from 980°C...

Table 3.16 Mechanical properties of some martensitic steels...

The need for heat treatment after forming or welding is a complex topic and can only be mentioned briefly here. Generally speaking, however, unless the forming operation has been very severe it is not necessary to heat treat to restore mechanical properties of austenitic types, although in special cases it may be advisable to do so to relieve stresses. With the martensitic steels... 

Austenitic steels of the 304S15 type are normally heat treated at 1 050°C and cooled at a fairly rapid rate to remove the effects of cold or hot working, and in this state much of the carbon is in supersaturated solid solution. Reheating to temperatures below the solution treatment temperature leads to the formation of chromium-rich MjjCj precipitates predominantly at the grain boundaries with the production of chromium gradients and reduced corrosion resistance as is the case with the martensitic steels. Any attack is... 

The occurrence of stress-corrosion cracking in the martensitic steels is very sensitive to the magnitude of the applied stress. For instance, a 13% chromium martensitic steel tested in boiling 35% magnesium chloride solution (125.5°C) indicated times to failure that decreased abruptly from more than 25(X)h to less than 0.1 h as the applied stress was increased from 620 MPa to about 650 MPa (Fig. 8.25). However, the effects of stress on time to failure are not always so dramatic. For instance, in the same set of experiments times to failure for a 17Cr-2Ni martensitic steel gradually decreased from more than 800 h to about 8 h as the applied stress was increased from 500 MPa to 800 MPa. 

Fig. 20.47 Light micrograph showing the microstructure of a martensitic steel (x 550)...

The martensitic alloys contain 12 to 20 percent chromium with controlled amounts of carbon and other additives. Type 410 is a typical member of this group. These alloys can be hardened by heat treatment, which can increase tensile strength from 550 to 1380 MPa (80,000 to 200,000 Ibf/in ). Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, freshwater, and organic exposures). In the hardened condition, these materials are very susceptible to hydrogen embrittlement. 

Without these advances in hard, strong materials based on abundant, and therefore low-cost iron ore, there could have been no industrial revolution in the nineteenth century. Long bridges, sky-scraper buildings, steamships, railways, and more, needed pearlitic steel (low carbon) for their construction. Efficient steam engines, internal combustion engines, turbines, locomotives, various kinds of machine tools, and the like, became effective only when key components of them could be constructed of martensitic steels (medium carbon). 

The Ms temperature, at which the diffusion-less martensitic transformation starts, depends on the alloy considered (its composition, etc.) it can be above or below room temperature. For the so-called austenitic steels Ms < < ambient temperature, whereas Ms > ambient temperature for the martensitic steels. 

EN 10222-2, Steel forgings for pressure purposes — Part 2 Ferritic and martensitic steels with specified elevated temperature properties. 

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