Estimation of Weld Metal Hardness of Cr-Mo Steels

Hardness variation in the Cr-Mo steel weld metal as a function of Cr content and weld cooling rate was studied Chromium in the weld metal was varied by adding known amount of high purity Cr into the weld pool produced by a stationary arc. The cooling rate of the weld was measured by plunging W-Rh thermocouple in to the weld pool just at the time of extinguishing the arc and measuring the variation in temperature as a function of time using a X-t recorder. The microstructural studies revealed that in the low Cr steels the structure varied from fully martensitic to a mixture of bainite and martensite as the weld cooling rate decreased while for weld metal with Cr content higher than 5 wt% the structure remained fully martensitic irrespective of the cooling rate. The measured hardness of the weld metal for different Cr contents and cooling rates was compared with that predicted by Yurioka's and Hart's formulae. It was found that Yurioka’s formula can be used to predict the hardness of the weld metal with reasonable accuracy. It was further noticed that there is a reduction in the maximum hardness at Cr content above 5wt% and it is attributed to the presence of delta ferrite in the weld metal.