A Metallurgical Understanding of Welding Electrode Specifications for High Strength Steels

Abstract

K Sampath

High-strength steel (HSS) welding electrode specifications offer two sets of Tables for compliance, one on Specified Electrode Chemical Composition Requirements and second on Specified Minimum Weld Mechanical Properties Requirements. These sets of Tables may appear mutually exclusive but underlying metallurgical principles keep them inter-dependent. Suppressing austenite transformation-start (TS ) temperature simultaneously increases both strength and low-temperature Charpy V-notch (CVN) impact (or fracture) toughness of HSS weld metal (WM).

Specifically, a two-step approach is useful in understanding the metallurgy of high-strength, high-performance electrodes and WMs. This approach includes calculated austenite transformation-start (TS ) temperatures such as Ar3 , BS or MS , besides carbon content, Yurioka’s carbon equivalent number (CEN) and balanced Ti, B, Al, N, O additions, that correlate identified WM chemical composition with desired high-performance microstructures to meet or exceed minimum WM tensile and CVN impact (or fracture) toughness property requirements.

The first step uses a set of constitutive (statistical/regression) equations to control the amounts of principal alloy elements such as C, Mn, Cr, Ni, Mo, and Cu so the relevant calculated austenite transformation-start (TS ) temperatures such as Ar3 , BS , or MS and Yurioka’s CEN stay in a desirable range relative to the base metals being joined. While doing so, one also needs to ascertain that the common progression of calculated austenite transformation-start (TS ) temperatures wherein Ar3 > BS > MS remains valid.

The second step requires balanced Ti, B, Al, N, O additions with a Ti/B ratio at 10:1, to further lower the actual austenite transformation-start (TS ) temperature compared to the calculated TS temperature. Both a lower austenite transformation-start (TS ) temperature and a narrow start-to-finish (TS –TF) temperature range for austenite decomposition ensure exceptional CVN impact toughness. The balanced Ti, B, Al, N, O content can be ascertained using an artificial neural network (ANN) template offered by the Japan Welding Engineering Society (JWES) at its website. A Ti/B ratio at 10 seems to allow exceptional CVN impact (or fracture) toughness, and (Ti+B+Al+N+O) additions between 500 ppm (0.05 wt-%) and 600 ppm (0.06 wt-%) is consistent with a lowering of austenite transformation-start (TS ) temperature. The JWES-ANN template allows one to manipulate 16 elements of the WM compositions, each within a specified range and seek a lower predictive temperature (T28J / °C) below -80 °C for achieving 28 J absorbed energy during CVN impact testing.

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