Effect of Weld Schedule on the Residual Stress Distribution of Boron Steel Spot Welds
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HARDENED boron steel (22MnB5) is classed as an ultra-high-strength steel (UHSS) consisting of a significant proportion of martensite, with an ultimate tensile strength (UTS) of up to 1500 MPa.[1] The steel has been utilized in the automotive industry in structural and anti-intrusion components such as B-pillars, bumper reinforcements, roof, and side rails.[1–3] The main attraction of using martensitic steels is the weight reduction, achieved through gage reduction, and increased passenger safety it provides. Boron steel in its as-delivered form consists of ferritic-pearlitic microstructure.[4,5] The as-delivered steel is usually austenitized at 1173 K and 1223 K (900 C and 950 C) for 5 to 10 minutes[5–7] and then quenched and formed in a single step in a die. Due to the high strength, press-hardened boron steel exhibits poor cold formability compared to other steel grades;[2] hence, a hot forming process is used to produce the martensitic phase transformation and desired part shape in one step. As mentioned, boron steel has found an important application in the automotive industry. A major joining method in the automotive industry is resistance spot welding, with several thousand welds made on a single
N.D. RAATH and D.J. HUGHES are with the WMG, University of Warwick, Coventry, CV4 7AL, UK. Contact e-mail: Neill. [email protected] D. NORMAN and I. MCGREGOR are with the Tata Steel Automotive Engineering, IARC Building, University of Warwick, Coventry, CV4 7AL, UK. R. DASHWOOD is with the Coventry University, Coventry, CV1 5FB, UK. Manuscript submitted October 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
car. During spot welding, the weld and surrounding material are exposed to a wide range of temperatures, from the melting point at the weld center to ambient temperature in the base material (BM). As a consequence, the resistance spot weld (RSW) exhibits varying microstructures with corresponding varying material properties. The area directly underneath the electrodes at the sheet–sheet interface experiences the highest temperature, where the metal exceeds melting temperature and rapidly solidifies upon cooling (due to the water-cooled electrodes), leaving a hard weld nugget. A decreasing temperature gradient extends outward from this area (in the length as well as thickness direction), resulting in altered microstructures, known as the Heat-Affected Zone (HAZ). Particularly for boron steel RSWs, there is a steep gradient of material properties in the HAZ.[8] RSWs of the UHSS steel family exhibit unique hardness profiles compared with lower strength grade steels. The RSWs are characterized with high hardness values in the nugget and outlying BM, and a sudden drop in the area between these regions, as indicated in Figure 1. The cause for this sudden drop has been established to be due to tempering of the parent martensitic microstructure.[9–11] The microstructural differences between the HAZ and nugget/BM are an influencing factor on failure; however, the location of the HAZ plays an important role as well.[12]
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