Cold Crack Susceptibility studies on High Strength Low Alloy Steel 950A using Tekken Test

25 | P a g e J a n u a r y 2 0 1 7 h t t p s : / / c i r w o r l d . c o m / Cold Crack Susceptibility studies on High Strength Low Alloy Steel 950A using Tekken Test V.Manivelmuralidaran, M.sakthivel, M.Balaji 1Assistant Professor, Department of Mechanical Engineering, Kumaraguru College of Technology, Coimbatore, India, 2Professor, Department of Mechanical Engineering, Info Institute of Engineering, Coimbatore, India , 3Associate Professor, Department of Mechanical Engineering, Kumaraguru College of Technology, Coimbatore, India, amritamani@gmail.com, sakthivelm1962@gmail.com, balaji.m.mec@kct.ac.in ABSTRACT


INTRODUCTION
HSLA steels are used widely for construction of large scale welded structures. HSLA steel is a type of alloy steel that provides better mechanical properties [1]. The prime advantages of these types of steels are combination of high strength and toughness. HSLA steels vary from other steels, they are specific mechanical properties intended for their service [2]. They have a carbon content between 0.05-0.25% to retain formability and weldability. The cold crack susceptibility tests have been extensively tried and proven cold cracking test procedures are available [3]. The cold cracking test procedures and diverse investigation methods may also serve to determine hot crack formation during welding [4]. This is frequently dependent solely on the achievable stiffness conditions and external loads and also dependent on the associated thermo mechanical effects during welding and cooling [5]. Depending on their execution, cold cracking tests provide qualitative results (crack/no crack) or quantitative results for the investigated material/filler material combinations [6].
The cold cracking test procedures can be classified according to the type of loading into self-restraint and externally loaded tests [7]. Self-restraint cold cracking tests imply a structural load of which the level depends on the constructional stiffness conditions of the specimen as well as on phase transformation [ 3,8]. The objective of study is to determine the effect of preheating temperatures on cold crack susceptibility of HSLA steel. Along with other factors, cooling rate plays an important role in determining final weld microstructure [9,10]. Thus, the preheating temperature was selected from the literature and fixed as 100ºC and 150ºC. Microstructural constituent plays an important role in deciding the weld metal toughness. [11,[16][17][18]. Mechanical properties of the welded joints are depend upon thejoint characteristics and thereby improvement of mechanical property is essential to have sound welds.

SPECIMEN PREPARATION FOR TESTING
The test material taken for the study is HSLA steel of SAE Grade 950A with thickness 12mm and dimensions of 200mmX150mm. The chemical composition for the material is shown in Table 1. The Filler material selected for the study is ER70S-D2 which is found suitable for welding HSLA steels. This filler electrode has been preferred because of low hydrogen content [12][13]. The electrode was dehydrated prior to welding and its chemical composition and mechanical properties are shown in Table 2 and Table 3 respectively. The filler metal can yield better properties while using Carbon dioxide (CO2) as the shielding gas. Because of high spatter in CO2, in the present analysis 80%Ar-20% CO2 mixture has been taken for shielding gas. GMAW process is used for the study. Figure 1 gives the GMAW experimental setup used for welding HSLA steel. The test pieces were preheated to 100ºC and 150ºC prior to welding.

Y-GROOVE TEKKEN TEST
Y-groove Tekken test is used to study the cold cracking susceptibility of the welded metal. Y-groove Tekken test specimens are prepared as shown in figure 2. The Tekken test was carried out as described in the standard DIN EN ISO 17642-2 [11,12]. Figure 3 gives the cross section of the test and auxiliary welds. The test specimens were arranged for the required gap of 2 mm between them. The auxiliary welding was made for 60mm in two ends. Then the test welding was carried out in single pass. After welding, the material was kept for normal cooling. The welded material was allowed for 48 hours cooling and the material was cut into the five sections for analyzing the cold cracks. Figure 4 gives the five testpieces cut from the test weld. The welding trials were conducted using the standard procedures. The levels of parameters were determined based on preliminary tests. The welding parameters for the test weld were 25Volts, 10mm/minute wire feed rate and 200inch/ minute table speed.

Fig 3: Cross section of test weld and auxiliary weld
I S S N 2321-807X V o l u m e 1 3 N u m b e r 3 J o u r n a l o f A d v a n c e s i n C h e m i s t r y 28 | P a g e J a n u a r y 2017 h t t p s : / / c i r w o r l d . c o m / Figure 5 shows the cross section of the test material after welding. The material was cut into five pieces in the test weld region and analyzed for the micro structural constituents and cold cracks.

RESULTS AND DISCUSSIONS
The welded specimen was allowed to be kept for about 72 hours and cut at different locations to study the microstructure and for cold cracking. Figure 6 gives the microstructure of the base metal HSLA 950A at 400X magnification.  Table 5 gives the metallographic results of Y groove tekken test. In the metallographic analysis of the welded sample the crack found in both weld metal and heat affected zone. Further analysis proceeds into the type of crack formed in welding. Two different cracks were found in the analysis. The first crack found in the heat affected zone propogated to weld metal. This type of crack is termed as Chevron type of crack. Figure 8a shows the grey scale image of the test weld cross section showing the crack at the weld metal at preheating temperature of 100ºC. The crack initiated from the heat affected zone tends to grow into weld metal and finally tend to fail the weld metal. The crack propagation was reduced due to the presence of acicular ferrite microstructure in weld metal. Another type of crack was found in the heat affected zone of the weld metal. In the same manner crack formation in heat affected zone also affected the weld metal strength. Figure 9 shows the grey scale image of the crack formed in heat affected zone at preheating temperature of 150ºC. Because of higher preheating temperatures, the weld metal has more acicular ferrite compared with the sample preheated to 100ºC. Hence, preheating of the base metal prior to welding is essential in improving the cold cracking resistance and thereby reduces the cooling rate [21][22][23][24]. Analysis of the cold crack susceptibility with short range of temperature is inconclusive and hence studies with higher range of temperature is necessary to conclude the effect of preheating temperature.

CONCLUSION
The performed experimental investigations on weldments of HSLA steel SAE grade 950A by Tekken test clearly shows that within the covered range of welding conditions cold cracking may occur in the weld metal. Cracks are found in the heat affected zone and in weld metal. The crack is surface crack and perpendicular to the residual solidification stress. From the microstructure analysis, it is observed that the acicular ferrite formed in the weld metal has greater cold cracking resistance [13][14]. It is viewed in the microstructure that those weld metals having good resistance to the cold cracking posses has acicular ferrite microstructure. The preheating temperature is also having more influence on the microstructure that can be varied to have a better weld metal with good mechanical properties. The higher the preheating temperature, higher is the resistance to cold cracking [20]. The weld metal is not susceptible to hydrogen induced cracking, if the metal is preheated to 150ºC. It has been analysed that the preheating temperature is to be increased so as to increase the cold cracking resistance. Conclsions can be drawn that wide range of preheating temperature are to be selected and varied for effective applications of research findings in future.