The susceptibility to solidification cracking of unstabilized and stabilized ferritic stainless steels was investigated using self-restrained Houldcroft, Modified Varestraint-Transvarestraint (MVT), and hot tensile testing. Five experimental steel grades comprising an unstabilized, two mono stabilized (Ti or Nb), and two dual stabilized (Ti + Nb), and two commercial unstabilized and a dual stabilized (Ti + Nb), and another dual stabilized containing-Mo alloy (nine different alloys in total) were used in this study. Seven steel grades comprising an unstabilized, two mono stabilized (Ti and Nb) respectively, three dual stabilized (Ti + Nb) and a dual stabilized containing Mo were used for the self-restrained Houldcroft method. Autogenous gas tungsten arc welding at a speed of 6 mm/s, 3 mm/s, and 1 mm/s was done. The unstabilized ferritic stainless steel was resistant to solidification cracking. Ti addition to ferritic stainless steel resulted in a minor increase to susceptibility to solidification cracking. Nb in ferritic stainless steel increased solidification cracking. The addition of Ti and Nb resulted in a decreased susceptibility to solidification cracking compared to an alloy containing only Nb. The weld metal microstructures were a mixture of columnar and equiaxed grains. The interdendritic crack surfaces were enriched in Nb, Ti, Mn, Si, Al, Mn, and Mo. The MVT test was used for the test of an unstabilized, a Nb stabilized and two (Ti + Nb) dual stabilized ferritic stainless steels. Two different welding speeds of 6 mm/s and 3 mm/s using autogenous gas tungsten arc welding were employed. The high content (Ti + Nb) steel at a welding speed of 3 mm/s had the greatest sensitivity to solidification cracking. The Nb stabilized steel at both welding speeds (6 mm/s and 3 mm/s) and high content (Ti + Nb) steel at a welding speed of 6 mm/s showed intermediate sensitivity to solidification cracking. The unstabilized and low content (Ti + Nb) grades were the least sensitive to solidification cracking. The weld metal microstructures transverse to the welding direction revealed columnar grains in all the samples for both welding speeds. Three experimental Ti-, Nb-, and dual Ti + Nb stabilized ferritic stainless steels were used for hot tensile testing using a Gleeble-1500D thermo-mechanical machine at testing temperatures of 1200°C, 1250°C, and 1300°C. The dual stabilized ferritic stainless steel showed a high and fairly constant hot ductility with an increasing testing temperature. The Ti stabilized alloy revealed a slightly lower ductility compared to the dual stabilized steel but much higher ductility than the Nb stabilized ferritic stainless steel. The SEM images of the intergranular cracking showed interdendritic morphologies. EDX analysis showed the elements Al, Mn, Ti, Si, Ni, S, Nb, and Ni to be associated with the fractured surfaces. The hot tensile test results were inconclusive, due to the small number of samples and an acquisition frequency that was too low. The MVT test was better than the self-restrained Houldcroft, and the self-restrained Houldcroft was better than the hot tensile tests in quantifying the susceptibility of a specific ferritic stainless steel alloy to solidification cracking. The cracking response of Houldcroft seemed to be dominated by welding speed. Cracking response of MVT test seemed to be dominated by the Nb content. The effect of Nb and Ti on the susceptibility cracking could be explained in terms of the effect of these two alloying elements on the difference between the liquidus and the solidus. Nb was found to segregate strongly to the grain boundaries (low k value) which resulted in a significant increase in the difference between the liquidus and the solidus. This difference increased BTR which results in a high susceptibility to solidification cracking. Ti has a higher k value and segregates less than Nb during solidification. / Thesis (PhD)--University of Pretoria, 2018. / Materials Science and Metallurgical Engineering / PhD / Unrestricted

Nickel super alloy Inconel 718 was tested and compared to Haynes 230 using Gleeble and Varestraint mechanical test methods. Hot cracking susceptibility was examined in either alloy using a sub-scale Varestraint test method at 5 augmented strain levels: 0.25%, 05.%, 1%, 2%, and 4%. Maximum crack length, total crack length, and number of cracks were measured for each strain level. Gleeble hot ductility on-heating and on-cooling tests were performed on both alloys. Inconel 718 was tested on-heating at target temperatures of 1600˚F, 2000˚F, 2100˚F, 2200˚F, and on cooling at 1600˚F, 1700˚F, 1800˚F, 1900˚F, and 2100˚F. Haynes 230 was tested on-heating at target temperatures of 2050 ˚F, 2200 ˚F, 2240 ˚F, 2330 ˚F, and on-cooling at 1800 ˚F, 1900 ˚F, 1990 ˚F, 2040 ˚F, 2090 ˚F, 2100 ˚F, 2140 ˚F, and 2190 ˚F. Ductility in Gleeble samples was measured in a reduction of surface area. A nil-strength temperature was established for either alloy. The nil-strength temperature was 2251˚F and 2411˚F, for Inconel 718 and Haynes 230, respectively. The nil ductility temperature <5% R/A) was 2188˚F for Inconel 718 and 2341˚F for Haynes 230. Ductility recovery temperature occurred at 1924˚F for Inconel 718 and 2147˚F for Haynes 230. The brittle temperature range was determined to be 326˚F for Inconel 718 and 228˚F for Haynes 230. Varestraint testing revealed that Inconel 718 had a lower threshold strain for crack initiation than Haynes 230 (0.5% vs 1%), and a higher number of cracks, as well as a larger maximum crack length, at every strain level. These results show a greater tendency for liquation cracks to form in Inconel 718 than in Haynes 230.

7039 aluminum alloys are widely being used in the aerospace, automotive and defense industries in which welding technique is used for their joining. The main problem encountered during the welding of 7039 aluminum alloy is hot cracking. The aim of this study is to understand the effect of welding parameters on the hot cracking behavior of 7039 aluminum alloy by using Modified Varestraint Test (MVT) with Gas Tunsgten Arc Welding (GTAW) technique. During tests, welding speed was selected as varying parameter, welding current was kept constant and to understand the effect of filler materials 5183 and 5356 aluminum alloy filler materials were used. It has been observed that with the change in welding speed hot cracking susceptibility of 7039 aluminum alloy changes. The effect of filler materials is found to be favorable by decreasing the hot cracking susceptibility of 7039 aluminum alloy. Filler material additions also improved the hardness of the weld metal. Based on the cracking mechanism hot cracks were investigated as solidification cracks and liquation cracks. It has been experienced that liquation cracking susceptibility of the filler material added samples has been affected from the magnesium and manganese contents of the weld seams. Effect of solidification range on liquation cracking was also justified with differential thermal analyses. With the micro examinations the intergranular structure of hot cracking is revealed. In addition, the characterization and growth properties of the hot cracks under cyclic load were tried to be understood and the fractography of these cracks were taken.

Nickel based superalloys were developed to withstand the severe thermal and mechanical environment associated with rocket propulsion systems and jet engines. In many alloy systems the strength of a component rapidly deteriorates as the operating temperature increases. Nickel based superalloys, however, retain strength over a range of temperatures which includes the operating range for many propulsion systems. This improved performance is accomplished by a combination of solid-solution strengthening, precipitation strengthening and grain-boundary strengthening. Furthermore, super-alloy systems are designed for ease of fabrication, to include machining, welding and heat treating. Inconel 718 was developed to overcome problems with post-weld cracking that were common in precipitation hardened nickel based superalloys strengthened by γ’. Inconel 718 is strengthened by γ’’ and is less sensitive to cracking during post-weld thermal treatment. However, in some cases, compositional changes which improved the behavior of these alloys during stress relief actually led to greater difficulty during the joining process. Many approaches have been used to determine the hot-cracking sensitivity of Inconel 718. Historically, two approaches have been particularly valuable because of their repeatability, their ability to compare different alloy systems and their verisimilitude to actual fabrication. These are the Gleeble hot-ductility test and the Variable-Restraint (Varestraint) weld test. Varestraint samples were prepared as per standard preparation techniques and tested longitudinally with a GTAW. At a predetermined location a strain was applied perpendicular to weld direction. The applied strain varied from 0.25%, 0.5%, 1.0%, 2.0%, and 4.0%. The Inconel 718 yielded a maximum crack length of 0.6 mm with a saturation strain of 2.0%. Both the total crack length and the number of cracks did not have a saturation strain. Gleeble samples were prepared from rod stock and tested with standard methodology to determine the characteristic temperatures: nil ductility, nil strength, and ductility recovery temperature of Inconel 718. The samples were tested at various pull temperatures on-heating until the nil strength temperature then tested on-cooling with the nil strength temperature acting as the peak temperature. The nil strength temperature was 2273°F, nil ductility temperature was 2182°F, and the ductility recovery temperature was 1925°F. Both the Varestraint and Gleeble results were compared with relevant literature to determine the weldability of the Inconel 718. Four criteria were used to determine the weldability of Inconel 718 and in three of the four tests; the Inconel 718 had equal to or greater weldability than the compared materials. In the fourth test, the Inconel 718 demonstrated lower weldability than the compared alloy systems, however, Inconel 718 operates in different conditions specifically, the high temperature and pressure conditions mentioned above.

Hot cracking is a serious problem that encounters during welding of aluminium-magnesium alloys. In the present study, solidification and liquation type of hot cracks in weld metal and the heat-affected zones of 5086-H32 aluminium alloy were investigated by using Modified Varestraint Test (MVT) with TIG-AC and TIG-DC welding. With determining the size, type and number of cracks, a relation was established between welding line energy and strain on the hot crack formation. This information was used to determine the hot crack safe parameter ranges. The hot cracking tendency as a function of applied parameters were discussed in the frame of temperature fields around the moving heat source. Moreover, the characteristic hot crack locations on the 5086-H32 MVT specimens were generalized. The results of the study indicated that the increase in line energy and strain increased the hot cracking tendency of the specified aluminium alloy. In the low line energy range, the main hot cracking mechanism is the solidification cracking which could be overcome by the use of a suitable filler material. At high line energy range, due to the increased amount of interdendritic liquid, the amount of solidification cracking decreases by healing mechanism. However, because of the enlarged-temperature-field around the weld zone, fraction of HAZ cracking increases. The comparison between the hot cracking tendencies in low and high line energies indicates that the low line energy ranges with low augmented strains resulted in hot crack safer parameters.

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