Embrittlement temper

For heat-resistant steels long-term service at temperatures above 400 °C (e.g., high-pressure steam pipes) can lead to a decline in impact strength. Normally, transition temperatures (precipitous decline of notched bar impact values) of below 0 °C are encountered, but this can increase to 60 °C and more (temper embrittlement). 

The susceptibility to temper embrittlement can be reduced by controlling the level of trace elements (Si, Mn, P, Sn) in steels [1241], [1242]. Vessels for which temper embrittlement is anticipated should not be pressurized below a certain temperature. 

As the name implies, the affected material disintegrates into fine metal and metal oxide particles mixed with carbon. Depending on the defects in a protective oxide film on the metal surface and the ability of the material to sustain this film, an induction period may be observed until metal dusting manifests itself as pitting or general attack. A possible mechanism was proposed by Grabke [1250] and Hochmann [1251]. 

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Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Mechanism of Temper Embrittlement, paper presented at the ASTM Symposium, Philadelphia, PA, October, 1970. 

Bruscato, R., Temper Embrittlement and Creep Embrittlement of 2.25Cr-lMo Shielded Metal-Arc Weld Deposits, Weld. J. 49, Res. Suppl., pp 148-s -156-s, April 1970. 

The major cause of temper embrittlement is segregation of phosphorus to the grain boundaries. A common way to minimize temper embrittlement of 21/4Cr-1Mo base metal is to limit the J" factor to a maximum of 180 where J is defined as ... 

Step cooling will not simulate embrittlement of 1 WCM Mb, though it occurs (e.g., a 100°F [38°C] increase in transition temperature was reported after 8 y at 930° F [500° C]). This is because the embrittlement in 11/iCr-1 Mo is caused by precipitation of carbides in the ferrite phase rather than segregation of impurities to the grain boundaries. Temper embrittlement can be reversed by heating at 1,150°F (620°C) for 2 h per inch of thickness. 

Brittle fracture is always a concern with heavy wall vessels. An 80 to 100°F (27 to 38° C) minimum hydrostatic test temperature is often specified to minimize the possibility of brittle fracture during hydrostatic testing. To minimize the possibility of brittle fracture of heavy wall reactors during start up and shutdown, reduced pressure below 200 to 300° F (90 to 150°C) is usually specified. Typical limitations are 40% of the design pressure or 20% of the original hydrostatic test pressure. With the advent of temper embrittlement resistant steels and weld metal, some refiners feel reduced pressure is only required below 100°F (38°C). 

The author discusses selected examples of Auger electron spectroscopy applications to the study of the role played by sulfur adsorption in the field of heterogeneous catalysis and its implication for the metallurgical problems of segregation, surface self-diffusion and temper embrittlement. 

Step Cooling This is a testing procedure only to evaluate the long term effects of temper embrittlement. The test was developed by GE originally for turbine blades and since adopted by the refining industry for hydroprocessing reactors. This test of temper embrittlement is used for 2-1/4 Cr materials only. The heat treatment takes about 12 days before testing of the coupon can occur. 

J Factor Since temper embrittlement is a function of alloy and tramp elements present in the steel. The J factor is based on a mathematical equation that combines the overall effects of the various ingredients that are to be controlled. For 2-1/4 Cr and 3 Cr the limit is typically 100. For 1-1/4 Cr it is about 180 maximum. It applies to all product forms except tubing. The equation is ... 

S] Hippsley, C. A., Hydrogen and Temper Embrittlement Interactions in Fatigue of 2 l/4Cr-lMo Steel," Harwell Laboratory Report AERE R 12322, Oxon, U.K., 1986. 

After tempering some steels are susceptible to temper embrittlement AUoy steels are most susceptible, especially the chromium-nickel steels. A molybdenum content of 0.5% or less, however, reduces the susceptibility and Mo is an important alloying element added to steels to minimize temper embrittlement... 

Temper EMBRin cement. The phenomenon of temper embrittlement occurs when hardenecarbon structural steel is K>led slowly or l ld within a critical range of temperatures below the temperature at which austenite is transformed to ferrite. This critical temperature range usually occurs in somewhere between 850 and 1100° F. Welded joints and at elevated temperatures are... 

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