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Embrittlement of steels
Embrittlement of steels
Most susceptible to the phenomenon of strain-age embrittlement are low-carbon rimmed or capped steels that are severely cold worked during forming processes. Subsequent moderate heating during manufacture (as in galvanizing, enameling, or paint baking) or aging at ambient temperature during service may cause embrittlement.
Rapid cooling, or quenching of lowcarbon steels (0.04 to 0.12% carbon) from subcritical temperatures above about 560°C can precipitate carbides within the structure and also precipitation harden the metal. An aging period of several weeks at room temperature is required for maximum embrittlement.
Bright steel surfaces oxidize to a blue-purple color when plain carbon and some alloy steels are heated between 230 and 370 °C. After cooling, there is an increase in tensile strength and a marked decrease in ductility and impact strength caused by precipitation hardening within the critical temperature range.
Quenched steels containing appreciable amounts of manganese, silicon, nickel, or chromium are susceptible to temper embrittlement if they also contain one or more of the impurities antimony, tin, and arsenic. Embrittlement of susceptible steels can occur after heating in the range 370 to 575 °C (700 to 1070 °F) but occurs most rapidly around 450 to 475 °C.
500 °F embrittlement.
High-strength low-alloy steels containing substantial amounts of chromium or manganese are susceptible to embrittlement if tempered in the range of 400 to 700 °F (200 to 370 °C) after hardening, resulting in tempered martensite. Steels with microstructures of tempered lower bainite also are subject to 500 °F embrittlement, but steels with pearlitic microstructures and other bainitic steels are not susceptible. ~,
400 to 500 °C embrittlement.
Fine-grained', high-chromium ferritic stainless steels, normally ductile, will become embrittled if kept at 400 to 500 °C (750 to 930 °F) for long periods of time. Soaking at higher temperatures for several hours should restore normal ductility.
Prolonged service at 560 to 980 °C (1050 to 1800 °F) can cause formation of the hard, brittle sigma phase in both ferritic and austenitic stainless steels and similar alloys. Impact strength is greatly reduced, particularly when the metal has been cooled to about 260 °C (500 °F) or less.
Formation of graphite may occur in a narrow heataffected zone of a weld in carbon and carbon-molybdenum steels held at temperatures over 425 °C (800 °F) for prolonged periods. The degree of embrittlement depends on the distribution, size, and shape of the graphite formed in the heat-affected zone.
Long exposure of galvanized steel to temperatures slightly below the melting point of zinc (420 °C or 787 °F) causes zinc diffusion into the steel. This results in the formation of a brittle iron-zinc intermetallic compound in the grain boundaries. Other types of embrittlement leading primarily to intergranular fracture are caused by environmental factors. These include the following:
Neutron irradiation of steel parts in nuclear reactors usually results in a significant rise in the ductile/brittle transition temperature of the steel. Metallurgical factors such as heat treat practice, microstructure, vacuum degassing, impurity control, and steel composition greatly affect susceptibility to this type of grainboundary weakening.
Hydrogen atoms diffuse readily into steel during processes such as acid pickling, electroplating, arc welding with moist or wet electrodes, and exposure to hydrogen sulfide. After stressing, delayed brittle fracture may occur, particularly in higherstrength steels.
Simultaneous exposure to a tensile stress (applied or residual) and to a relatively mild corrosive environment may cause brittle fracture in metal parts that may be either intergranular or transgranular, depending on conditions. If either factor is eliminated, stress-corrosion cracking cannot occur.
Certain liquid metals can embrittle the solid metals with which they are in contact. A tensile stress is also required for brittle fracture to occur.
Each of the above types of embrittlement is the result of exposure to one or several environmental factors during manufacture, storage, or service. Each type is extremely complex and must be considered during any failure-analysis investigation.
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