The Effect of Martensitic Phase Transformation Dilation on Microstructure, Strain–Stress and Mechanical Properties for Welding of High-Strength Steel

Abstract

The application of low transformation temperature (LTT) wire can effectively reduce residual stress, without the need for preheating before welding and heat treatment after welding. The mechanism reduces the martensitic transformation temperature, allowing the martensite volume expansion to offset some or all of the heat-shrinking, resulting in reduced residual stress during the welding process. In this paper, commercial ER110S-G welding wire and LTT wire with chemical composition Cr10Ni8MnMoCuTiVB were developed to solve the problem of stress concentration. The microstructure of the LTT joint is mainly composed of martensite and a small amount of residual austenite, while the microstructure of the ER110S-G joint is mainly composed of ferrite and a small amount of granular bainite. The micro-hardness and tensile strength of the LTT joint is higher than that of ER110S-G joint; however, the impact toughness of the LTT joint is not as good as that of the ER110S-G joint. The martensitic phase transformation of LTT starts at 212 • C and finishes at around 50 • C, and the expansion caused by phase transition is about 0.48%, which is much higher than that of the base metal (0.15%) and ER110S-G (0.18%). The residual tensile stress at the weld zone of the ER110S-G joint is 175.5 MPa, while the residual compressive stress at the weld zone of LTT joint is −257.6 MPa.2 of 15 reduced tensile residual stress during welding process. This not only reduces the production cost, but also improves the fatigue resistance of the welded components [9].Dai et al.[10] used the traditional commercial OK75.78 wire and the low transformation temperature (LTT) wire to weld two comparable Weldox960 base plates. The residual stress of the joints were measured using neutron diffraction, with the results showing that LTT can effectively reduce the welding residual stress of high-strength steel welding and improve the stress distribution. Chiaki and Shiga [11] found that the residual stress distribution is ideal when the starting temperature of martensite transformation is 200 • C, and the residual compressive stress at the weld toe is 300 MPa.Ohta et al. [12] found that by using the 10 Cr-10 Ni-0.7 Mn wire the fatigue limit of the LTT joint could be twice that of conventional joints. Barsoum et al. [13] studied the effect of residual stress on fatigue strength. Fatigue tests showed that the LTT joint had a 40% increase in fatigue strength compared to conventional joints. Fabrice et al. [14] found that the LTT wire significantly improved the fatigue strength of the joint, and the fatigue strength was 60% higher than that of the conventional wire (G3Si1) filling joint in two million cycles. Wang et al.[15] compared the conventional wire of C-Mn and LTT wire of 9.1 Cr-8.5 Ni-1.25 Mn, and showed that the fatigue strength was improved by 59% when using LTT wire. Altenkirch et al.[16] simulated the stress state of the welded structure during service and analyzed the changes in its organization and performance. The results showed that the low transfor...

Surface- and volume-based investigation on influences of different Varestraint testing parameters and chemical compositions on solidification cracking in LTT filler metals

“…In particular, the proof that compressive residual stresses are formed [3,4], the investigation of the mechanisms of stress formation [5,6], and the effect on the fatigue strength [7,8] have been the subject of many research projects. Recent publications also deal with extended topics such as microstructure and the associated mechanical properties [9][10][11][12][13], the behaviour during multilayer welding [14][15][16][17][18], and the application of LTT in beam welding [19].…”

Microstructures and Properties of Martensitic Materials

“…Martensite is the product of a diffusionless shear-like transformation. The papers published in this Special Issue [1][2][3][4][5][6][7][8][9] confirm that research in martensitic materials is very active. Even the old 'simple' Fe-C alloys keep many secrets that are still beyond the capabilities of modern computers.…”

“…Welding alloys needs specific and careful tests in order to take into account the residual stresses. Chen et al [8] show that, by designing the alloy to reduce the transformation temperature, the martensite volume expansion can offset the usual tensile stresses generated by heat-shrinking; and this reduces the distortion between the welded joints and improves the performances of the welds. As mentioned previously, martensite is not restricted to steels; and other important martensitic materials are shape memory alloys.…”

Application of Probabilistic Approach to Investigate Influence of Details in Time History of Temperature Changes on the HCF Life of Integrated Bridge Steel Piles Installed on Water

“…This process also has an effect on the welding joints and leads to the elimination of tension residual stress (TRS) caused by the welding process [27]. However, the Almen intensity as one of the most effective parameters of shot peening treatment is very important because the martensitic phase is formed in the HAZ (i.e., material is extremely brittle in this area) [28,29]. In this way, the incorrect selection of shot peening parameters causes the formation of cracks in the welding area and ultimately reduces the life of the structure.…”