Effect of Tempering Process on Microstructure and Mechanical Properties of Microalloyed Steels Joined by Submerged Arc Welding

Abstract

The main benefit of microalloyed steels is to provide important energy and cost savings in the manufacturing of forged components for automotive applications. In such steels, the strength levels and other properties achieved after cooling from hot working temperatures are reported to be comparable with those obtained from conventional quenched and tempered steels. Microalloying or the use of additions of elements (V, Nb, and Ti) at small rates in low carbon steels has been successfully employed for large diameter pipelines, bridges, and other construction applications. This has been extended to medium carbon steels for various automotive engine and engineering applications. The microalloying elements produce precipitation of carbonitrides in austenite, and the pro-eutectoid ferrite and pearlite phases of the final microstructure in order to obtain grain refinement and precipitation strengthening. Submerged arc welding (SAW) is extensively used in industry to fabricate pressure vessels, pipelines, marine vessels and wind turbine towers. The extensive use of SAW is associated with certain inherent metallurgical and process advantages such as (a) high deposition rate (18 kg/h) and high electrode deposition efficiency (b) low nitrogen in weld metal (c) excellent weld bead appearance, and (d) possibility of welding over a wide range of thickness. In addition to high productivity, absence of smoke, arc flash, and thus minimum protective clothing make this process find a prominent place in fabrication industries. Microalloyed steel in dimension of 400x200x6 mm was used for welding in this study. Table 1 illustrates the chemical compositions of the asreceived metals and Oerlikon-S2 submerged arc welding wire used for joining. Before welding, microalloyed steels were heat treated at 100 o C and than the samples were centred as face to-face with 1-mm interval, The welding process was performed as two passes from front side and then back side by using Oerlikon Magmaweld brand ZD5-1000 B model submerged arc welding machine. In this work, microalloyed steels joined by using submerged arc welding method under 450 A welding current parameters. Tempering process was performed to welded joint. Welded joints were characterised in terms of hardness, tensile testing before and after tempering. The results indicated that microalloyed welded joints showed the lower hardness, yield and tensile strenght after tempering whereas the increase in % elongation. While the breaking in the HAZ at the welded joint which was not applied tempering process on the contrary breaking was observed at the base metal after tempering process. It was observed that the hardness of the welded samples S1 and S2 raised when the measurement was carried out along the horizontal measurement line from as-received metal to HAZ or weld metal. The hardness measurement was also carried out along the vertical measurement line for weld metal obtained by two passes. It was observed that the hardness values gradually decreases from second weld pass to the first weld pass. This situation shows that second weld pass applies stress relief annealing on first weld pass and therefore the hardness decreases accordingly figures 5 and 6.

Keywords

• Microalloyed Steel, Submerged Arc Welding Method, Mechanical Properties.

Temperleme İşleminin Tozaltı Kaynak Yöntemi ile Birleştirilen Mikroalaşımlı Çeliklerin Mekanik Özelliklerine Etkisi

Abstract

Bu çalışmada, mikroalaşımlı çelik malzemeler tozaltı kaynak yöntemi ile 450 A kaynak akım değeri kullanılarak birleştirilmiştir. Kaynaklı bağlantıya temperleme işlemi uygulanmıştır. Temeperleme öncesi ve sonrası olmak üzere numunelere sertlik ve çekme testi uygulanmıştır. Sonuç olarak, mikroalaşımlı çelik malzeme kullanılarak yapılan kaynaklı bağlantıların temperleme sonrası daha düşük sertlik, akma ve çekme dayanımı gösterirken buna karşın % uzamada artış gösterdiği tespit edilmiştir. Temperleme uygulanmayan kaynaklı bağlantılarda kopma ısının tesiri altında kalan bölgesinde (ITAB) meydana gelirken temperleme sonrası kopma ana malzemede olduğu görülmüştür.

Keywords

• Mikroalaşımlı çelik, Tozaltı Kaynak Yöntemi, Mekanik özellikler