Evaluation of High Temperature Properties and Microstructural Characterization of Resistance Spot Welded Steel Lap Shear Joints

Introduction

In the last two decades, the usage of stainless steel materials has increased tremendously in various industrial applications. Stainless steel sheets are increasingly used for vessels, kitchen, transportation, building, etc. due to their high corrosion resistance, good appearance and good weldability [1–4]. Austenitic stainless steel constitutes the largest part among stainless steel family, in terms of alloy type and usage due to excellent combination of strength, corrosion resistance, formability and weldability [3–8]. Though weldability is good, due to sensitization phenomenon in the heat affected zone (HAZ) corrosion resistance of weld metal deteriorates [9–11] and therefore proper care is taken during welding.

For sheet metal joining, resistance spot welding (RSW) is an inexpensive and effective method. It is used extensively for joining low carbon steel components. High strength low-alloy steel (HSLA), stainless steel, nickel, titanium, aluminum and copper alloys are also spot welded commercially [12–16]. In the spot welded joints, metallurgical changes and mechanical property variation are possible, which depend on the load applied during welding and squeezing time. Selection of optimum parameters to achieve desired weld nugget and strength is very important especially when the joint is made for high temperature application.

Though several studies [12–16] have been carried out in spot welding of steels, its high temperature application potential has not been explored. In the present work, spot welding parameters are selected after extensive trials and after welding of several weld coupons. Different combinations of sheets are welded. After welding of sheets, mechanical test are carried out at various temperatures (under rapid heating condition). Microstructural analysis of different specimens is carried out, which provides new insight on spot welded joints of stainless steels.

Materials and experimental methods

Austenitic stainless steel AISI 304 sheets of 0.5/0.6 mm thickness and 17-4PH stainless steel sheets of 0.5/0.6 mm thickness were used for spot welding. Both sheets were cut into coupons of dimension 100 × 20 × 0.5/0.6 mm. Sheet materials were lap spot welded using 200 kVA pneumatic spot welding machine of British Federal make.

Welding was carried out by using water cooled cylindrical Cu-Cr electrodes. Welding was performed by overlapping the sheets linearly to fabricate the specimens for lap shear tensile test. Schematic of weld set-up is presented in Figure 1.

Figure 1:

Schematic diagram of spot welding of stainless steel sheets.

For joining, 6–8 kA welding current was applied, while other welding parameters such as applied electrode pressure and holding time of electrode were kept constant as given in Table 1. In order to determine the tensile shear load bearing capacity of welded materials, one set of three test specimens were prepared to carry out the test at each test temperature (Figure 1). It was determined by using a microprocessor controlled Zwick-type servo hydraulic universal testing machine. High temperature test fixture was made to hold the specimens under tensile load. To simulate the heating rate, specimens were directly loaded at the desired temperature and then after 30 sec, tensile load was applied and specimens were tested. K-type thermocouple was used to measure the actual temperature of test specimen.

The transverse sections of the weld passing through the weld nugget as well as similar sections of base plates were prepared by standard metallographic technique. 10% oxalic acid was used for etching through electrolytic process. Microstructure was observed under an Olympus make microscope.

Results and discussion

Chemical composition of stainless steel sheet used is presented in Table 2. The study was conducted to generate data for fin assembly of small rocket experiencing a typical temperature profile (Figure 2). Accordingly different combinations of weld joints were prepared and tested at various temperatures where temperature profile of fin assembly component was simulated by providing near equal amount of time in testing at specific temperatures.

Figure 2:

Temperature history on a typical fin assembly of small rocket using steel and PC 10 (polymeric thermal protection coating) + steel.

Photograph of spot welded sheet is presented in Figure 3. Visual observation made on the spot welded joints showed that bead shape and size were uniform. Subsequent test at different temperatures also had confirmed the absence of premature failure of joint. Mechanical properties evaluated for different combinations of joint materials and at various test temperature are presented in Tables 3–5 and Figure 4.

Figure 3:

Photograph of spot welded and tested specimens.

Figure 4:

Variation of failure load (mean) with increasing temperature.

From Table 3, it is clear that definite decrease in tensile strength of parent metal takes place when exposed to higher temperature. This reduction in properties of 17-4PH stainless steel is found to be higher, since steel is precipitation hardenable and strengthening by precipitates is reduced at elevated temperatures due to either coarsening of precipitate or dissolution of precipitates at still higher temperature. In fact, properties of 17-4PH steel and 304 stainless steel become nearly equal at 1,200 K test temperature. This clearly indicates that strengthening is through matrix of alloy only at that temperature. Reduction in strength of austenitic stainless steel at higher temperature is due to grain coarsening and softening of matrix at elevated temperatures.

Spot welded joints tested at room temperature show, strength comparable to parent metal for the same alloy. Also, failure load is found to be higher when thickness of the joint at one side is higher. However, when combination of 17-4PH and SS-304 is spot welded and tested, properties are found to be almost near to the properties of spot welded joint of SS-304. This clearly indicates that thickness of SS-304 decides the properties of joint. Failure location also is found to be in SS304 side of the parent metal. However, when sheet thickness of SS-304 is increased by 0.1 mm, marginal increase in overall strength is also noted.

It is interesting to note from the Tables 4 and 5 and Figure 4 that spot welded strength of 17-4PH steel at elevated temperature (800 K) is almost ~90% of the property at room temperature for the respective combination of joints. This trend is maintained up to 1,000 K and then it decreases drastically at 1,200 K test temperature. It clearly indicates that other than parent metal strength, spot weld strength is very high due to fast solidification of weld nugget, which may produce martensitic microstructure at the weld. But in case of SS-304 weld, gradual decrease in strength is noted as the test temperature increases from 800 K to 1,200 K. Here structural changes does not occur in the weld, only the solidified fine dendritic structure gets homogenized at different test temperature to various extent and accordingly strength varies. Further, properties at 1,200 K are found to be nearly same for all the combinations of joint. This indicates that at 1,200 K all the precipitates/martensitic structure dissolve (in 17-4PH steel joint) and convert back to austenite. One more significant observation has been noted i.e. spot weld strength is higher than the parent metal strength tested at the same temperature (Tables 3 and 5). This may be due to double the thickness at the weld joint and also the presence of martensitic phase at the spot weld, which helps to increase the failure load of joints higher than parent metal. Failure location is found to be in parent metal, near to the weld, indicating it is weaker area. In case of dissimilar joint of 17-4PH and SS-304, failure has taken place toward SS-304 side due to lower strength.

Optical microstructures of parent metal as well as spot welded joint cross section are observed. Photomicrographs of tested specimen cross section are presented in Figures 5–8. It is observed that 17-4PH steel shows primarily tempered martensitic structure, which is present even with short term exposure to 1,200 K. SS-304 shows presence of austenitic structure with some grain coarsening at 1,200 K temperature. Figure 6(a) shows a typical spot welded microstructure consisting of three zones, weld nugget, heat affected zone (HAZ) and base metal.

Figure 5:

Optical photomicrographs representing (a–c) 17-4PH and (d–f) SS-304 sheet at different test temperatures (a, d) 800 K, (b, e) 1,000 K, (c, f) 1,200 K.

Microstructural gradient near HAZ is clearly seen. The material in HAZ experiences grain coarsening also. However base metal microstructure away from HAZ remains unchanged. Weld nugget is found to be free from discontinuities. This confirms that electrode force was appropriate. Due to small difference in Cr and Ni contents in the two steels at the joint and as both alloys are getting welded, at the weld nugget a new composition is developed, which has different microstructure due to composition change through interdiffusion and also due to solidification. Rapid cooling of the weld nugget leads to high temperature gradient, and grains try to grow in a direction perpendicular to solid/liquid interface and form cellular dendritic structure owing to epitaxial growth mechanism. Weld nugget has columnar type structure and grains are found to be elongated parallel to electrode direction.

Figure 6:

Optical photomicrographs representing lap joint of (cross section) 17-4PH at different test temperatures (a, b) 800 K, (c)1,000 K, (d) 1,200 K.

Weld nugget microstructure (in 17-4PH steel) is found to be mainly martensitic due to high rate of cooling [17]. This is found to be similar for the two combinations of joints (17-4PH to 17-4PH and 17-4PH to SS 304). When the weld joints are tested at elevated temperatures, it is observed that martensitic phase formed due to rapid cooling of weld nugget gets tempered/converted to austenite (Figure 6(b–d)) under the influence of temperature with stress. In the case of stainless steel 304 (Figure 7), fine dendritic solidified structure starts homogenization at 800 K. This may be due to relatively lower Ms and Mf temperatures and accordingly formation of martensite is restricted except where localized carbon content is higher (Figure 7). Weld joints tested at 800 K (7a, b) shows partial homogenized dendrite of spot weld, which is further homogenized and become coarse at 1,000 K and 1,200 K test temperature (Figure 7(c and d)). Coarsening of austenitic grains of parent metal is clearly visible and formation of zones can be seen across the weld nugget at 800 K to 1,200 K (Figure 7(b–d)). Disappearance of multiple zones available at 800 K and 1,000 K is clearly seen at 1,200 K tested samples. It shows zonal structures formed due to differential cooling across the weld nugget get homogenized at 1,200 K temperatures.

Figure 7:

Optical photomicrographs representing lap joint of (cross section) SS-304 at different test temperatures (a, b) 800 K, (c)1,000 K, (d) 1,200 K.

Further, when dissimilar metal joint through spot welding is made, weld nugget is observed (Figure 8(a)) to be similar to SS-304 nugget having multiple zones, which slowly disappear during testing at higher temperature (Figure 8(b)–(d)). Width of the zones varies in welds of same metal joint (Figures 6 and 7) and dissimilar metal joint up to 1,000 K, unless zones are properly homogenized. In fact, homogenization process is faster here due to combined effect of temperature and stress with differential composition. Width of HAZ is found to be varying along the thickness and width of the weld. It is higher in the direction of electrode and lower perpendicular to it. This indicates that heat transfer mainly takes place across the thickness of sheet. It is also true since in the width direction, sheets do not have continuity of metal due to weld geometry. Across the thickness of weld, differential microstructure is seen in parent metal confirming austenitic SS-304 steel at the top and martensitic 17-4PH steel at the bottom.

Figure 8:

Optical photomicrographs representing lap joint of (cross section) 17-4PH and SS304 at different test temperatures (a, b) 800 K, (c) 1,000 K, (d) 1,200 K.

Concluding remarks

Spot welded joints of similar metal and dissimilar metal (SS-304, 17-4PH) on thin sheet were made and the joints were tested with exposure at elevated temperatures of 800 K, 1,000 K and 1,200 K to simulate fin assembly of small rocket experiencing this temperature regime in flight condition. Following conclusions have been made.

1. Spot welding process parameters are found to be almost the same for similar as well as dissimilar steel joints of SS-304 and 17-4PH sheets.

2. Tensile strength of spot welded joints is found to be of the order of >80% of the room temperature and the same has been maintained up to 1,000 K for 17-4PH steel. However, drastic reduction in strength is noted at 1,200 K, indicating dissolution of precipitates/tempering of matrix/conversion to austenite.

3. Tensile failure load of spot welded joints is found to be of the order of ~70% of the room temperature, and the same has been maintained up to 800 K for SS-304 steel and gradual reduction in strength is noted unlike in the case of 17-4PH steel, indicating coarsening of austenitic matrix.

4. Dissimilar metal joint failure load is found to be ~60% of the strength of 17-4PH steel, which is slightly higher than SS-304 steel up to 1,000 K. At 1,200 K, failure load is found to be almost the same as that of the similar metal joint.

5. Irrespective of parent metal, joint failure load at 1,200 K is found to be similar in all combinations of joints of SS-304 and 17-4PH. This indicates that at 1,200 K, role of matrix alone is significant where matrices of both the steels give similar strength.

6. In all the cases, failure is in the parent metal, near the weld nugget. In case of dissimilar metal joints failure is found to be toward SS-304 steel side.

7. Microstructure of weld cross section clearly shows presence of three zones, weld nugget, HAZ and base metal. HAZ width is found to be varying and is higher toward metal thickness direction.

8. Microstructure of weld nugget (17-4PH containing joint) is found to be martensitic, which gets tempered/converted to martensite at high temperature test of 1,200 K. Grain orientation inside the weld nugget is found to be in the direction of electrode indicating direction of heat transfer also. In case of SS304 weld joint microstructure consists of fine solidified dendrites, which homogenizes at test temperatures.

9. Spot welding failure load also indicates that if application temperature is up to 1,000 K, benefit of 17-4PH is significant and at 1,200 K benefit of using 17-4PH steel is marginal with respect to strength.