The effect of welding on reliability of mechanical properties of AISI 1020 and AISI 6150 steel materials

2.1. Materials

Two types (AISI 1020 and AISI 6150) of steel sheets were used in the study. In order to examine the effect of welding on the reliability of mechanical properties, these metal sheet plates were used both welded and without welds. Beside of this, these steels were chosen at different weldability levels (Odebiyi et al., 2019Odebiyi, O.S., Adedayo, S.M., Tunji, L.A., Onuorah, M.O. (2019). A review of weldability of carbon steel in arc-based welding processes. Cogent Eng. 6 (1), 1609180. https://doi.org/10.1080/23311916.2019.1609180.). Welding parameters were given in the following section. Spectral analyzes of the samples were performed before the experiments. The chemical compositions of steel sheets are shown in Table 1.

Specimens were machined from steel sheets and tensile test specimens prepared. Shape and dimensions of tensile specimens are shown in Fig. 1.

After machining, the specimens were normalized and mechanical tests of normalized specimens were done. Normalizing was accomplished at certain temperature above the transformation range and into the range of complete austenite according to Carbon content (900 °C for AISI 1020 and 850 °C for AISI 6150).

2.2. Process parameters of welding

In order to observe the reliability of widespread welding applications in the industry, the welding process was applied manually. Welding parameters have been selected based on their current applications in mutual assessment with the organization operating on the welding sector. In addition, the studies in the literature were taken as a basis in the selection of welding parameters (Abbasi et al., 2012Abbasi, K., Alam, S., Khan, M.I. (2012). An Experimental Study on the Effect of MIG Welding parameters on the Weld-Bead Shape Characteristics. Engineering Science and Technology: An International Journal (ESTIJ) 2 (4), 599-602.; Ibrahim et al., 2012Ibrahim, I.A., Mohamat, S.A., Amir, A., Ghalib, A. (2012). The effect of Gas Metal Arc Welding (GMAW) processes on different welding parameters. Procedia Eng. 41, 1502-1506. https://doi.org/10.1016/j.proeng.2012.07.342.; Odebiyi et al., 2019Odebiyi, O.S., Adedayo, S.M., Tunji, L.A., Onuorah, M.O. (2019). A review of weldability of carbon steel in arc-based welding processes. Cogent Eng. 6 (1), 1609180. https://doi.org/10.1080/23311916.2019.1609180.) Gas Metal Arc Welding (GMAW. Gas metal arc welding (GMAW), Gas Tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) were used to identify different welding methods to steel sheets. Welding parameters are shown in Table 2.

2.3. Mechanical tests and micro-structural investigation

Tensile and Vickers micro hardness tests were performed on the samples and microstructural investigations of samples were done. Tensile tests were performed on by using Autograph Shimadzu AG-IS 100 kN universal test machine according to ISO 4136:2012 (2012)ISO 4136:2012 (2012). Destructive tests on welds in metallic materials - Transverse tensile test. Testing and Inspection of Welds. standard. Vickers micro hardness values were measured with Future-Tech FM-700 micro hardness test machine by loading 100 g of force for 10 seconds. The results of the tensile experiments were evaluated with reliability analyses. Also micro structural investigation was done to relate obtained tensile and hardness results. Steel sheets were cut and specimens prepared following standard procedures for micro structural investigation. Samples were etched with Nital solution (3 vol. %NH₃ and 97 vol. % Alcohol). Nikon LV 100 optical microscope and Nikon SMZ 1000 stereo microscope were used for microstructural investigation.

2.4. Reliability analyses

In reliability analysis, distributions of random variables are determined, and reliability analyzes of the data obtained are performed through these distributions (Pham, 1994Pham, H. (1994). Handbook of Reliability Engineering. Springer. https://doi.org/10.1002/9780470172414.). The most widely used model for the distribution of a random variable is a Gauss (normal) distribution. When the number of random experiments increased, the probability distribution tends to have a normal distribution. Beside of this, Weibull distribution is one of the other most used distributions to model of many different physical systems as the parameters in the distribution provide a great deal of flexibility and ability to fit data from various fields. (Horst, 2008Horst, R. (2008). The Weibull Distribution - A Handbook. CRC Press.; Montgomery and Runger, 2014Montgomery, D.C., Runger, G.C. (2014). Applied Statistics and Probability for Engineers. John Wiley & Sons, Inc.). For instance the strength distribution curve for the ductile materials is described as Normal distribution, while that for the brittle material is Weibull distribution (André Meyers, 2009André Meyers, M. (2009). Mechanical Behavior of Materials 2^nd Ed. Aircraft Engineering and Aerospace Technology. Vol. 81, No 2, Emerald Group Publishing Ltd. https://doi.org/10.1108/aeat.2009.12781bae.001.). In this study, reliability analysis of test results was done to observe welding on mechanical reliability of welded parts. In previous studies, it was seen from statistical distribution analysis of welded steel materials that, Gauss (Normal) distribution adequately describes most of the mechanical properties except for the hardness and toughness (Saoudi et al., 2017Saoudi, A., Khamouli, F., Atoui, L., Zidani, M., Farh, H. (2017). Statistical distribution analysis of mechanical properties of a welded pipeline steel API X70 and correlation between hardness and other mechanical characteristics. Int. J. Eng. Res. Africa 30, 49-64. https://doi.org/10.4028/www.scientific.net/JERA.30.49.) serial mechanical properties of 70 pipes, formed by spiral submerged arc welding of high strength low alloy steel (HSLA. Also there are some previous studies using the Weibull distribution in the reliability studies of the mechanical properties of welded samples included tensile tests (Colombo et al., 2019Colombo, T.C.A., Rego, R.R., Otubo, J., de Faria, A.R. (2019). Mechanical reliability of TWIP steel spot weldings. J. Mater. Process. Tech. 266, 662-674. https://doi.org/10.1016/j.jmatprotec.2018.11.021.; Ku et al., 2019Ku, M.H., Hung, F. Y., Lui, T.S. (2019). The effect of hyper-rotation on the Weibull distribution of tensile properties in a friction stirred AA7075 aluminum alloy. Mater. Chem. Phys. 226, 290-295. https://doi.org/10.1016/j.matchemphys.2018.12.085.; Yang and Jiang, 2019Yang, C.W., Jiang, S.J. (2019). Weibull statistical analysis of strength fluctuation for failure prediction and structural durability of friction stirwelded Al-Cu dissimilar joints correlated to metallurgical bonded characteristics. Materials 12 (2), 205. https://doi.org/10.3390/ma12020205.). In reliability analysis of this study, Shapiro-Wilk normal distribution test was applied to all the test results. Tensile results were found to be suitable for normal distribution and hardness tests results were found to be not suitable as expected. Reliability analyses according to Weibull distribution were examined and compared. In addition, normal and cumulative distributions of Vickers hardness results were obtained with the acceptance of Gauss distribution in order to show the wide range and low reliability as a result of welding process. The equations of these distributions were given under below. Eq. (1) is Gauss distribution and Eq. (2) is Weibull distribution.

 (1)

 (2)

In equations, x: Random variable, : Average deviation of x, Sx: Standard deviation, b: Shape parameter of Weibull function, θ: Scale parameter of Weibull function, f(x): Distribution function of x, F(x) = ∫f(x).dx: Cumulative distribution function of x. The results of the experimental data and statistical analysis were included in the results and discussion section.

3.1. Tensile test results

Tensile test results are given in Fig. 2. It can be seen in Fig. 2 that ultimate tensile strength of welded parts has a more widespread distribution. This is a sign that the welding process affects the reliability of the mechanical properties. With the welding process, the standard deviation increased, and this increased the range of data obtained.

It was also seen from Fig. 2 that tensile strengths of welded specimens were decreased. The decrease in the tensile strength of the samples is the result of the inhomogeneous welding microstructure (Yang and Jiang, 2019Yang, C.W., Jiang, S.J. (2019). Weibull statistical analysis of strength fluctuation for failure prediction and structural durability of friction stirwelded Al-Cu dissimilar joints correlated to metallurgical bonded characteristics. Materials 12 (2), 205. https://doi.org/10.3390/ma12020205.). Micro hardness test results and microstructural investigation also confirmed this thought. When evaluated in terms of optimum welding method for both steel materials; the welding method in which tensile strength decreases minimum was SMAW welding in AISI 1020 steel and GMAW welding in AISI 6150 steel. Shielded arc welding is an important process which is widely used in industry sector for joining process etc. AISI 1020 is widely used by all industry especially in manufacturing sector. SMAW welding method is one of the widest welding method for this material (Shukla et al., 2018Shukla, B.A., Joshi, V.S., Chel, A. E,M., Shukla, B.A. (2018). Analysis of Shielded metal arc welding parameter on Depth of Penetration on AISI 1020 plates using Response surface methodology. Procedia Manufacturing 20, 239-246. https://doi.org/10.1016/j.promfg.2018.02.035.). Consequently, it was seen the study that, as a perspective of tensile strength, the method is appropriately can be used for welding method of AISI 1020 steel.

AISI 6150 steel is a fine grained, highly abrasion-resistant chromium vanadium alloy steel, which has high strength, high fatigue strength and large hardenability. In recent years, AISI 6150 steel has been commonly used in many industries, particularly the automotive industry (Li et al., 2013Li, Hy., Hu, Jd., Li, J., Chen, G., Sun, X.J. (2013). Effect of tempering temperature on microstructure and mechanical properties of AISI 6150 steel. J. Cen. South Univ. 20 (4), 866-870. https://doi.org/10.1007/s11771-013-1559-y.). AISI 6150 steel is extensively used in large-size damping springs for trains and vehicles (Çivi et al., 2018Çivi, C., Yurddaskal, M., Atik, E., Celik, E. (2018). Quenching and tempering of 51CrV4 (SAE-AISI 6150) steel via medium and low frequency induction. Mater. Test. 60 (6), 614-618. https://doi.org/10.3139/120.111196.; Zhang et al., 2018Zhang, L., Gong, D., Li, Y., Wang, X., Ren, X., Wang, E. (2018). Effect of quenching conditions on the microstructure and mechanical properties of 51CrV4 spring steel. Metals 8 (12), 1-16. https://doi.org/10.3390/met8121056.). Despite such widespread use, there is limited study in the literature about welding methods of AISI 6150 steel, (Mendes et al., 2013Mendes, R., Ribeiro, J.B., Loureiro, A. (2013). Effect of explosive characteristics on the explosive welding of stainless steel to carbon steel in cylindrical configuration. Mater. Design 51, 182-192. https://doi.org/10.1016/j.matdes.2013.03.069.; Yalamaç et al., 2016Yalamaç, E., İnceli, U., Karaagaç, M., Yildiz, S. (2016). Characterization and Analysis of Welding Area Between Alloy Steel Main Part and Bended End Part. Çukurova University Journal, Faculty of Engineering and Architecture 31 (1), 89-97. https://dergipark.org.tr/en/download/article-file/301666.). Hardly any studies on tensile tests of welded AISI 6150 steel have been found. In this study, it has been seen that GMAW has given better results for this material with a slight difference. As known, the gas shield in SMAW is not clean enough especially for some materials. On the other side, GTAW and GMAW are very clean welding processes. The main advantage of GMAW over GTAW is the much higher deposition rate, which allows thicker workpieces to be welded at higher welding speeds. The skill to maintain a very short and yet stable arc in GTAW is not required (Kou, 2002Kou, S. (2002). Welding Metallurgy. Second Edition, John Wiley & Sons, Inc.). These reasons are thought to have an impact on the results obtained.

3.2. Vickers micro hardness test results

The microstructure hardness was measured from center of the welded zone to base material. The results were given in Fig. 3 and Fig. 4. It was seen the figures that, welding zone, heat-affected zone (HAZ) and base material have different hardness values as expected.

Comparing the base metal with the weld metal and the HAZ, a hardness increment was observed. It is estimated that the reason for this increase is martensite formation with reference to previous studies (Delgado et al., 2016Delgado, J.A., Ambriz, R.R., Cuenca-Álvarez, R., Alatorre, N., Curiel, F.F. (2016). Heat input effect on the microstructural transformation and mechanical properties in GTAW welds of a 409L ferritic stainless steel. Rev. Metal. 52 (2), e.068. https://doi.org/10.3989/revmetalm.068.). Another point to be noted here is that the hardness level, which is almost the same in non-welded samples, contains large differences in the regions mentioned in welded samples. It is also seen that different welding methods have different effects on the samples in terms of hardness distribution. For instance, in AISI 1020 specimens, GTAW and GMAW welding methods provide great differences in hardness distributions while GMAW welding method provide different hardness distribution from other welding methods for AISI 6150 specimens. This shows that different welding methods directly affect the dimensions of the welding and HAZ region. When evaluated in terms of hardness results, it can be concluded that the SMAW method, whose distribution values for both materials do not differ greatly, is more suitable with reference to previous studies (Yalamaç et al., 2016Yalamaç, E., İnceli, U., Karaagaç, M., Yildiz, S. (2016). Characterization and Analysis of Welding Area Between Alloy Steel Main Part and Bended End Part. Çukurova University Journal, Faculty of Engineering and Architecture 31 (1), 89-97. https://dergipark.org.tr/en/download/article-file/301666.). However, tensile tests and microstructure analyzes should also be evaluated for the selection of the most suitable welding method.

3.3. Micro structural investigation

To understand the effects of the welding process, examining the microstructure of the main materials and welded joints is one of the important issues (Zdravecká and Slota, 2019Zdravecká, E., Slota, J. (2019). Mechanical and microstructural investigations of the laser welding of different zinc-coated steels. Metals 9 (1), 91. https://doi.org/10.3390/met9010091.). The results of microstructural analyses were given in Figs. 5-8Figs. 5, 6, 7, 8. Figure 5 shows macro photographs of welded structures taken with a Stereo microscope. The discontinuity of the welded structure can be clearly seen in the photographs. It can be commended that, heat input made for the welding would create a microscopically heat affected region.

Figures 6, 7 and 8 show microstructural photographs of non-welded and welded structures taken with optical microscope.

Hardness distributions (Figs. 3-4) were directly related to microstructure photographs (Figs. 6-8Figs. 6, 7, 8) as expected. The relatively homogeneous microstructure in the non-welded material provided a uniform distribution of hardness. Microstructural differences in the welding metal and in the heat affected zone are directly related to the standard deviation obtained with the high hardness distribution. As a result of the microstructural investigation it was seen that, In AISI 1020 steel; considering the weld penetration, heat effect of the welding and welding seam, SMAW is the most suitable method. On the other hand, in the microstructure studies for AISI 6150 steel, it was observed that the better microstructure conditions that will provide better strength values are formed by GMAW welding.

3.4. Reliability analysis

The distribution of tensile and hardness test results under consideration of normal distribution were given in Figs. 9-10. Left side of the figures show obtained tensile (maximum strength) and hardness values and right side show the cumulative distribution of tensile and hardness values.

The reliability analysis of the data obtained in the material tests is of great importance in both the test accuracy and the evaluation of the material manufacturing parameters (Çivi et al., 2014Çivi, C., Tahrali, N., Atik, E. (2014). Reliability of mechanical properties of induction sintered iron based powder metal parts. Mater. Design 53, 383-397. https://doi.org/10.1016/j.matdes.2013.07.034.; Çömez et al., 2019Çömez, N., Çivi, C., Durmuş, H. (2019). Reliability evaluation of hardness test methods of hardfacing coatings with hypoeutectic and hypereutectic microstructures. Int. J. Miner. Metall. Mater. 26 (12), 1585-1593. https://doi.org/10.1007/s12613-019-1866-x.; Gül et al., 2019Gül, C., Çömez, N., Çivi, C., Durmuş, H. (2019). Reliability Analysis of Brinell Hardness Results for Aged Alumix321/SiC Composites. Trans. Indian Inst. Met. 72 (9), 2311-2318. https://doi.org/10.1007/s12666-019-01681-5.) iron based powder metal parts (3% Cu, 0.5% Graphite and 1% Kenolube lubricant by weight. It can clearly be seen from Figs. 9-10 that, the distribution exhibited a narrow gap on non-welded specimens while it exhibited wide gap after welding. And also, in 1020 steel, which is low hardenability steel because of its low carbon content, the hardness distribution does not show a wide range after welding, while the hardness range is very wide as expected in 6150 steel (Mendes et al., 2013Mendes, R., Ribeiro, J.B., Loureiro, A. (2013). Effect of explosive characteristics on the explosive welding of stainless steel to carbon steel in cylindrical configuration. Mater. Design 51, 182-192. https://doi.org/10.1016/j.matdes.2013.03.069.; Maharjan et al., 2020Maharjan, N., Zhou, W., Wu, N. (2020). Direct laser hardening of AISI 1020 steel under controlled gas atmosphere. Surf. Coat. Tech. 385, 125399. https://doi.org/10.1016/j.surfcoat.2020.125399.). Narrow distributions refer to high reliability values. When the standard deviations and the range of values obtained increase, the probability of an obtained value of reflecting the entire cluster decreases, so the reliability values decrease.

It was seen from the previous studies that, both Weibull and Normal distribution can be used as statistical distribution of tensile strength of welded and non-welded materials. However, Weibull distribution can be used especially in a small number of experiments, since it is a more flexible distribution and can also meet the normal distribution in some cases. In this study, tensile test results were found suitable for normal distribution, but Vickers test results were found not suitable as expected. Because of this, Weibull distribution was used to carry on reliability analyses of Vickers hardness results. Curves of the Vickers hardness test results are also shown in the Figs. 11-12.

When Vickers hardness results are examined, it was seen that the hardness distribution can be provided by the Weibull distribution due to microstructural differences in different parts of the welded structure in parallel with previous studies in the literature (Saoudi et al., 2017Saoudi, A., Khamouli, F., Atoui, L., Zidani, M., Farh, H. (2017). Statistical distribution analysis of mechanical properties of a welded pipeline steel API X70 and correlation between hardness and other mechanical characteristics. Int. J. Eng. Res. Africa 30, 49-64. https://doi.org/10.4028/www.scientific.net/JERA.30.49.) serial mechanical properties of 70 pipes, formed by spiral submerged arc welding of high strength low alloy steel (HSLA).

As a result of the all tests and analyzes, it was seen that all the mechanical properties values of the welded samples were decreased after welding process. Welding method is a commonly used joining and repair method in construction structures. When we evaluate in terms of optimum welding method for these two steel materials; the welding method in which tensile strength decreases the least is SMAW in 1020 steel and GMAW in 6150 steel. According to non-welded samples, HAZ and base metal difference and regional hardening occurred in welded samples with the effect of heat. For instance, in the AISI 6150 steel, the hardening is 289 HV in the non-welded sample and 488 HV in the MIG welded sample. As a result, SMAW in AISI 1020 steel; It has been found to be the best method in terms of welding penetration, welding heat effect, welding seam image and arc welding application cost. Considering all these factors for AISI 6150 steel, GMAW is the best welding method regarding the conditions in this study and the data obtained.