Indian Welding Journal

S. K. Albert ¹, T. P. S. Gill ¹, V. Ramasubbu ¹, S. D. Kulkarni ²

Affiliations
1 Materials Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, IN
2 Indian Institute of Technology, Mumbai-400 076, IN

Abstract

In this paper results obtained from diffusible hydrogen (HJ measurement and hydrogen assisted cracking susceptibility (HAC) of three different Cr-Mo steel welds are discussed. H₀ measurements were carried out using a gas chromatograph and HAC susceptibility studies using the ur-modified hydrogen sensitivity test (UT-Modified HST). Specimens for both these studies were prepared by employing identical welding parameters. Results showed that with increase in alloy content the cracking susceptibility of the steel increased while H₀ content in the weld decreased. The decrease in the H₀ content is explained based on decrease in the apparent diffusivity and increase in the apparent solubility of hydrogen with alloy content. These are in turn attributed to increase in the density of hydrogen traps with alloy content. Regression analysis of the results showed that it is possible to predict the safe preheat temperature for the steel from the composition if H₀ content in the weld is known.

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S. K. Albert ¹, T. P. S. Gill ¹, P. Rodriguez ¹, S. L. Mannan ¹, S. D. Kulkarni ²

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam, IN
2 Indian Institute of Technology, Powai, Mumbai, IN

Abstract

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.

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S. K. Albert ¹, T. P. S. Gill ¹, V. Ramasubbu ¹, F. C. Parida ¹, S. D. Kulkarni ²

Affiliations
1 Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, IN
2 Dept, of Met, Engg. and Mat, Sci., l.l.T. Powai, Mumbai 400 076, IN

Abstract

Hydrogen Assisted Cracking Susceptibility of 2.25Cr-1Mo steel was studied using UT-Modified Hydrogen Sensitivity Test (UT-Mod. HST). Welding was carried out using GTAW process and hydrogen was introduced into the weld metal by mixing it with argon shielding gas. Autogenous bead-on-plates were made on specimens fixed on a copper fixture and the specimens were strained for 4% strain immediately after welding for 24 hrs and checked for cracks. Preheating during welding was achieved by heating the copper fixture and the minimum preheat temperature required to prevent cracking for known vol.% of hydrogen in the shielding gas found out. Diffusible hydrogen content (H₀,) in the specimen prepared under identical conditions employed for testing was measured using Gas Chromatograph Weld cooling cun/es were obtained by plunging a thermocouple into the molten weld pool and cooling time (t_8/5)was estimated from the curves. Variation in weld metal hardness and microstructure was also studied. The results showed that both minimum preheat temperature and (H₀) content increased with hydrogen in the shielding gas. Preheating results in reduction in (H₀) content but does not affect the hardness indicating that cracking is prevented due to reduction in (H₀) content in the weld prepared with preheating. The results show that it is possible to choose an optimum preheat temperature to prevent cracking if (H₀) level in the weld is known.

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