Hydrogen embrittlement AISI 4340 steel

It is possible to provide cathodic protection to base plate up to 1 720 MNm yield strength, by coupling to mild steel or possibly to zinc , but zinc and metals more active than zinc tend to induce hydrogen embrittlement. Welds up to 1 380 MNm may be cathodically protected by zinc, but at impressed potentials of — 1-25V (S.C.E.) both 1240 and 1380 MNm welds fail rapidly due to hydrogen embrittlement. Neither mild steel nor zinc couples protect AISI 4340 steel . 

Analysis methods for hydrogen absorbed in the deposit have been described (65), and instruments are commercially available to detect hydrogen in metals. Several working tests have been devised that put plated specimens under strain and measure the time to failure. A method for cadmium-plated work has been described (66) as has a mechanical test method for evaluating treatments on AISI 4340 Steel (67). Additional information on testing for hydrogen embrittlement is also available (68). 

Figure 11.18 Reduction in area as a function of the strain rate for type AISI 4340 steel subjected to cathodic polarization in seawater leading to hydrogen embrittlement (o) polarization of 0.76 V ( jpolarization of 0.61 V. The grey zone shows values typically found in air.

Martensitic steels are generally susceptible to see in a wide range of environments. The martensite steels, such as grade AISI 400 types, are susceptible to hydrogen embrittlement when stressed or exposed to sulfide or chloride environments. The specific anions to cause failure are not as important as hydrogen. 

This equation reduces to Eh = -4.308 x 10 5T ln/n, i.e. Eh = -0.029 log/h at 20°C, where 0.98 bar hydrogen pressure would result from a hydrogen overvoltage of 29 mV. It is interesting to note that maximum embrittlement occurs here at a fugacity of 108 (i.e./fj = 104) compared with the much lower value of/h = 104 found for AISI 4340 steel by Scully and Moran,9 i.e. a martensitic microstructure is more readily embrittled by cathodic polarization than the duplex. 

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