Carbon steel Weldments - Factors influencing Hardness

By shreemurali Date 04-28-2008 06:25

I am involved in welding 1" thk CS pipes (with Max Ceq: 0.43) with internal cladding of Alloy 625. The filler metal is Alloy 625, the process is Machine GTAW and it is a single sided full-penetration weld.

While, there is no concern of the hardness in the weld (as the weld is deposited using Alloy 625), the focus shifts to the HAZ hardness values.

As the cooling rates from 800 deg C to 500 deg C, essentially determine the hardness, how does GTAW parameters compare with a process like SMAW? Is it logical to assume that the GTAW process due to the lower heat input values can provide a narrower HAZ compared to SMAW and hence lower hardness for a given Ceq of base metal? Or in other words, is it correct to say welding processes with higher heat input can result in wider HAZ widths and higher hardness?

I did not have a specific problem related to hardness while running the PQR`s as the hardness limits were as specified by ISO 15156 (Maximum hardness of 248 Hv10 in root and 275 Hv 10 in cap). But I would like to know from the peers about the very general concept of hardness of HAZ I have in mind is correct or not.

Thanks

By 803056

Date 04-28-2008 13:36

Lower heat input does result in a narrower heat affected zone, but that also promotes faster cooling rates that result in increased hardness if all other conditions are held constant.

Best regards - Al

By GRoberts (***)

Date 04-28-2008 19:14

1st off, I would caution against assuming GTAW is a low heat input process. It can be (especially with automation as you are using), but due to low travel speeds, the heat input can get very high indeed. However, low heat input does not generally help getting low hardness. As already pointed out, the higher cooling rates associated with low heat input contribute to high hardness. Also, HAZ widths are not really relavent to hardness testing. While the HAZ width is indicitive of the relative heat input (on the same type and thickness of material), there is not necessarily a correlation with hardness.