Study of the occurence of reversible temper embrittlement in the coarse-grained overheated structure of the heat-affected zone of steel 22 NiMoCr 37

On four melts of steel 22 NiMoC 37 with about 0.22% C,0.3% Si, 0.9% Mn, 0.5% Cr, 0.78% Mo and 1% Ni, but different contents of phosphorus, sulphur, arsenic, copper, antimony and tin, the toughness behaviour in fine-grained quenched and tempered condition and also in coarse-grained overheated condition has been investigated in the notched-bar impact bend test as a function of the cooling rate after tempering or stress relieving. In fine-grained condition, the toughness behaviour changed only slightly. In the coarse-grained condition, the transition temperature increased considerably with increasing contents of trace elements and decreasing cooling rate. With 0.015% P, 0.012% As and 0.006% Sn, the increase of the transition temperature in the coarse-grained condition and at an average cooling rate of 30 K/h was with ΔTsub(trans) = 25 K still relatively low. With higher contents of phosphorus and tin, however, an increase of the transition temperature up to ΔTsub(trans) = 145 K was observed. Copper alone did not lead in the performed test series to any impairment of the notched-bar impact energy. In coarse-grained condition, temper embrittlement was proved also metallographically by etching with xylene picric acid solution. Segregation lines proved to be especially susceptible. In temper embrittled condition, the notched-bar impact bend test specimens showed an intercrystalline fracture appearance in the lower shelf and the transition range of the notched-bar impact energy-temperature curves. The examination of such fracture surfaces with the Auger probe revealed in an about 10-4 mm large grain boundary region the expected concentrations of the elements phosphorus, tin and nickel, but also an enrichment of copper. Especially the elements phosphorus and tin contributed to the reversible temper embrittlement of the coarse-grained overheated structure. (orig.)