The base metal (BM) and the heat affected zone (HAZ) of a resistance spot welded dual phase steel have been evaluated by nanoindentation hardness testing. Three different surface conditions have been explored on the BM for assessing the nanohardness response. Softening has been investigated along the sub-critical HAZ by making nanoindentations on individual phases such as ferrite and tempered martensite (TM) at various distances from the line of lower critical temperature Ac1. A broken appearance accompanied with sub-micron particles were consistently found on TM at 100 μm from the Ac1 line suggesting carbide precipitation along with partial recovery of martensite. The morphology of TM kept on changing while moving away from Ac1 towards the BM as the fraction of broken appearance was reduced and the sub-micron particles became finer. SEM observations resulted in good agreement with the nanohardness of the TM phase along the sub-critical HAZ. In contrast, microhardness results suggested the termination of tempering at a shorter distance with respect to Ac1 and hence a reduced extension of the softening region. The improved resolution for assessing softening through nanoindentation was due to the possibility of avoiding the contribution of the phase boundaries because of the smaller size of the indentation; this also permitted evaluation of TM at low peak temperatures far from Ac1 where early stages of tempering took place.

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A study on heat affected zone softening in resistance spot welded dual phase steel by nanoindentation

Martensite–austenite (M–A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M–A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M–A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M–A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M–A constituent and carbon concentration on the slender M–A constituent is higher than that on the massive M–A constituent. Meanwhile, the distribution profile of silicon on the M–A constituent shows an obvious inhomogeneity. Most of M–A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M–A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.

Analysis of martensite–austenite constituent and its effect on toughness in submerged arc welded joint of low carbon bainitic steel

Influence of microstructural aspects on impact toughness of multi-pass submerged arc welded HSLA steel joints

Super-duplex stainless steels present excellent combination of mechanical and corrosion resistance, due to their strict composition control and ferrite–austenite phase balance This balance may, however, be disturbed during welding in both the fusion and HAZ due to the rapid cooling rates and may lead to loss of the good corrosion and mechanical properties of the weldments The present investigation is studying the effect of nitrogen addition in the plasma operation gases and of the increase of nickel in the filler metal, on the microstructure and on the mechanical properties of super-duplex stainless steels welded by the plasma transferred arc (PTA) technique Results have shown that nitrogen addition in the plasma operation gas affects the mechanical properties of the weldments It is shown that nitrogen addition in the plasma and protective gas and higher nickel content in the filler metal have both a positive effect on the elongation of the welded specimens and after optimization of the welding parameters very good results may be obtained in terms of tensile strength

Effect of nitrogen and nickel on the microstructure and mechanical properties of plasma welded UNS S32760 super-duplex stainless steels

Strengthening effect of nickel and copper interlayers on hybrid laser-TIG welded joints between magnesium alloy and mild steel

Effect of copper content on the microstructure and mechanical properties of multipass MMA, low alloy steel weld metal deposits

Effect of chromium content in the range of 0.05–0.91 wt% on the microstructure and mechanical properties of Cr–Ni–Cu low alloy steel weld metal was investigated. All welds were prepared by manual metal arc welding technique in flat position. Microstructure of the welds was examined by optical and scanning electron microscope in both columnar and reheated regions of the weld metal. The results showed increase in acicular ferrite and microphases formed at the expense of primary ferrite and ferrite with second phase with steady refinement of microstructure. According to these microstructural changes, yield and ultimate tensile stresses, Hardness and Charpy V-Notch impact toughness increased, whereas elongation decreased. Increase in Charpy impact value is thought to be due to fine dispersed spheroidized dark microphases at high chromium contents.

A. Haerian

Papers

Effect of copper content on the microstructure and mechanical properties of multipass MMA, low alloy steel weld metal deposits

Effect of chromium content on the microstructure and mechanical properties of multipass MMA, low alloy steel weld metal