Prevent Cracking in Deposition of Carbon Steel on Inconel 625

Welding procedure of clad steel including deposition of carbon steel on nickel base alloy usually gives unaccepted mechanical properties. Cracks were formed along type II boundary in nickel base alloy pass and a martensitic layer was formed in carbon steel pass. In this paper, cracks along type II boundary were prevented by lowering the martensitic start temperature (TMs) of the martensitic layer. Decreasing of TMs was obtained by two methods: Dilution method and Grain refining method. Three levels of TMs (approximately 350, 200, and 50C) are obtained. The results showed that: cracks along type II boundary were prevented at TMs lower than 200 C; however type II boundary itself was prevented at TMs lower than 50C. Also post weld heat treatment was necessary to achieve accepted impact properties.


Introduction
Welding of carbon steel pipes (X65) cladded by nickel base alloy (Inconel 625) are usually being welded by AWS A5.14-ERNiCrMo3 filler metal. A trial was attempted to weld first and second passes by AWS A5.14-ERNiCrMo3 and subsequent passes by AWS A5.1 E7018. Unaccepted mechanical properties were resulted due to creation of cracks along type II boundary and formation of martensitic layer in carbon steel deposit [1]. In the present paper an unconventional idea was developed to prevent cracking along type II boundary. This idea is illustrated schematically in Fig1, where martensite which formed in carbon steel pass (3 rd pass) was induced compressive stresses on 2 nd inconel pass; hence cracks were prevented. It must be mentioned that the idea of promoting martensitic formation in welds was considered a departure from conventional thinking [2]. But in the present work martensite would be beneficial if lower martensite start temperature was achieved. Lowering martensite start temperature (T Ms ) means that more compressive stresses were induced at surrounding passes i.e. transformation induced compressive stresses generation [2][3][4]. T Ms can be controlled by chemical composition and by grain size [5][6][7].
Two methods were used to reduce T Ms of the 3 rd pass. The first one was described as "Dilution Method". In this method, dilution level was increased by increasing the welding current. This means that more alloying elements (mainly Ni and Cr) would transfer from the 2 nd pass (Inconel) to the 3 rd pass (carbon steel).
These alloying elements will increase the hardenability and decrease T Ms . The second method was described as "Grain Refining Method". In this method T Ms decreased by decreasing grain size of the 3 rd pass. The composition of the 3 rd pass was similar to martensitic stainless steel [1]. Thus, tempering is necessary to provide the required notch toughness [8].
The post weld heat treatment (PWHT) was applied for crack free welds. Schematic diagram illustrated the idea of using martensite which formed in 3 rd pass to prevent crack along type II boundary in 2 nd pass: "X" longitudinal, "Y" transverse and "Z" through thickness 2 Experimental Procedures

Welding and Material
Base metal was API 5L Grade X-65 pipe steel with 305 mm diameter, 21.4 mm thickness and cladded with 2 mm thickness of inconel 625. Filler metals used in this study were AWS A5.14 ERNiCrMo3 and AWS A5.1E7018. The test coupon pipes were fabricated with full penetration single groove butt joint with 60 degrees included angles. Welding was done using flat position "1G". Gas tungsten arc welding (GTAW) process with pure argon shielding gas was used to weld first and second passes using AWS A5.14-ERNiCrMo3 filler metal. The heat inputs of the 1st and 2nd passes were lowered as possible (about 0.9KJ/mm) to minimize dilution.
Two methods were used to weld the 3rd pass. The first one was described as "Dilution Method". In this method; the 3rd pass was welded by shielded metal arc welding (SMAW) process using AWS A5.1 E7018. Three levels of dilution were achieved by using three levels of heat input (about1.2, 1.4 and 2.0 KJ/mm). The increase in heat input levels was obtained by the increase in welding current from 95 A to 110A and 150 A, respectively. The subsequent passes were welded by AWS A5.1 E7018 using the same heat input of the 3 rd pass. The second method was described as "Grain Refining Method". In this method, the 3rd pass was welded by flux cored arc welding (FCAW) process using E70T-4 with amperage of 230 A.. The subsequent passes were welded by SMAW using AWS A5.1 E7018 and applying 1.2 kJ/mm heat input.

Post Weld Heat Treatment
Tempering was proceeded at 720 o C and 3 hours holding time. Tempering was applied only for crack free welds

Microstructural Characterization
Specimens were cut and prepared for mechanical tests in accordance with ASME-Section IX , where tensile, impact and bend test are required. The specimens for impact test were prepared from cap and root (including the 3 rd pass). Microstructural characterization was performed using optical metallography and scanning electron microscope. Because of the wide range of compositions and microstructures, a number of chemical etchants were used. Nital (2 mL HNO3 and 98 mL ethanol) was used to reveal martensitic structure; Vilella's reagent (5 mL HCl, 1 gram picric acid, and 100 mL ethanol) was used to reveal grains of martensite. Mixed acids (equal parts of HCL, HNO3, and acetic acids) were used to reveal grain boundaries of nickel base alloy. Microhardness across transition region was measured using diamond pyramid indenter in conjunction with both 10 and 100 gram loads.

Determination of Martensite Start Temperature (TMs) for the 3rd Pass
In this investigation, T MS was calculated using empirical equations. For dilution method, Gooch equation (Eq.1) [9] was used. This equation is usually used for martensitic stainless. However, for grain refining method; Lee equation (Eq. 2) [10] was used. This equation considers the effects of chemical composition and austenite grain size on martensite start temperature. (2)

Where G is the ASTM austenite grain size and elements in weight fraction
Typical EDX detector was used to measure chemical composition of the 3 rd pass. But this detector can detect a limited range of X-ray energies so light elements (Z<10) such as carbon, nitrogen cannot be measured accurately. To overcome this problem; in the present work; estimated complete chemical composition was determined using back calculation methodology as follows: 1. Major alloying elements such as Fe, Ni and Cr were determined by EDX analysis. Then dilution levels of major elements were calculated using Eq. 3 [6].
Where D is the dilution, C w , C f and C b are the concentration of each element in weld metal, filler metal, and base metal respectively.
2. Average dilution level (D av ) of the major alloying elements was determined.
3. Eq.4 was used to calculate the concentrations of other alloying elements

Effect of TMs value on type II grain boundary conditions
It is accepted that grain boundary type II worked as a weak line which easy cracked [11,12]. In the present work martensitic transformation which formed in 3rd pass was used to produce compressive stresses on 2nd pass, hence tensile stresses were reduced and cracks were prevented. Because of elastic modulus increase with decreasing of metal temperature; the amount of compressive stresses generated from martensitic transformation increased with decreasing of TMs [13,14].
Because of quantitative measurements of stresses at type II grain boundary are hard to conduct, a qualitative method was used to give an indication about stress level. This technique was proceeded by observing any cracks near the fusion boundary i.e. observation of cracks means high tensile stresses. Thus an approach of the relation between TMs values of the 3rd pass and stresses level at fusion boundary was built. Depending on this methodology effective levels of TMs were approximately determined as the following: a. Case I of Dilution Method: heat input was about 1.2 KJ/mm which gave 6.5% average dilution. Mechanical properties are shown in Table 1 Mechanical properties are shown in Tables 1, 2 and 3. As shown in Table 4, TMs for the fusion zone of the 3rd pass equaled to 197 o C. Fig.3 shows type II grain boundary appeared without any interfacial cracks. This means that at 197 o C, the created compressive stresses were sufficient to overcome the most of tensile stresses and cracks were prevented. This observation reflects the resulted accepted side bend test and improved notch toughness (Table2, and 3 [15,16], the tensile stress which worked as a driving force for type II boundary formation, was nil. Table 4 shows that TMs for fusion zone of the 3rd pass was 48oC. Based on these results, it can be concluded that: at TMS equal or lower than 48 o C the generated compressive stresses at fusion boundary were sufficient to overcome all tensile stresses so type II boundary itself disappeared.  However, a plan view of 2nd pass is illustrated in Fig.5, where parts from filler metal are observed. Parts from filler metals were forced inside the 2 nd Inconel pass and solidified giving martensitic islands. Micro-hardness of these regions was ranged from 497 to 522 HV. These results are supported by SEM and EDX analyses in Fig.6 where iron percentage was about 92%. These islands were obtained as a result of using high welding current [17,18]. As shown in Fig.7 cracked type II boundary is observed parallel to theses filler metal islands. As shown in Table 4, TMs of martensitic islands within 2nd pass is 310 o C. This means that: at 310 o C compressive stresses were very low compared with tensile stresses so cracking occurred.
Although type II boundary near fusion boundary were prevented, formation of filler metal martensitic islands with cracks within 2nd Inconel pass leaded to poor impact toughness and unaccepted side bend results. These results are given in Table2 and Table3 respectively.   6. Case III of dilution method -SEM and EDX for filler metal islands within the 2 nd Inconel pass Fig. 7. Case III of dilution method-cracked type II boundary parallel to filler metal islands within the 2 nd Inconel pass One argument is that a refinement of the austenite grain size leads to the Hall-Petch strengthening of austenite, thereby making it difficult for martensite to form [22]. Grain refinement of the 3 rd pass was achieved by using E70T-4 which acted as a source of aluminum oxide and aluminum nitride. Aluminum nitrides and aluminum oxides which may be considered as non-metallic inclusions and impair mechanical properties were used here as nucleation sites causing grain refining. Fig. 8 shows the prior austenite grains of the 3 rd pass which is martensite. Grain size was measured using intercept Method -ASTM E112 giving ASTM grain size number equals to12.
As shown in Table 4, T Ms of the 3 rd pass is 56 o C, which was calculated using Lee equation. Fig. 9 shows that planer solidification region and type II grain boundary are disappeared where cellular structure is continued until fusion boundary. This means that: at 56 o C compressive stresses created at fusion boundary were sufficient to overcome all tensile stresses so type II boundary itself disappeared. Preventing formation of type II boundary was reflected on mechanical properties of weld metal, where accepted side bend test results and improved notch toughness (23 Joule 0 o C) are noted in Table2 and Table3 respectively.

Effect of Post Weld Heat Treatment
It is well known that the accepted notch toughness of carbon steel at 0 o C is 27 Joule [22]. However, in the as welded conditions, unaccepted notch toughness at 0 o C was resulted as shown in Table1. Thus in order to obtain accepted notch toughness, tempering was necessary. Tempering was proceeded at 720 o C for 3hr holding time. Based on the results taken from as welded conditions, tempering was applied only for crack free cases (case II of dilution method and grain refining method). The results of mechanical properties are represented in Table  5.  Fig.10a and Fig.11a give a plan view for fusion boundary between 2nd and 3rd pass for case II dilution and grain refined methods respectively. Tempered martensite was noted in the 3rd pass and dark etched region was found at the interface between 2nd pass inconel and 3rd pass (martensite). Micro hardness was illustrated in Fig.10b and Fig.11b where highly localized hardness peak was noted. As documented in literatures [23][24][25][26], this dark layer was enriched carbide layer which formed due to carbon migration from ferrite side to austenite side. This layer was clearly observed in grain refining method than dilution method due to difference of carbon content (0.22% and 0.07% respectively). The effect of this layer on notch toughness is also noted in Table 5. For case II of dilution method; notch toughness increased from 22 to 43Joule (as welded and tempered condition respectively). However for grain refining method notch toughness increased only from 23 to 30Joule (as welded and tempered condition respectively).

Conclusions
Based on the results and discussion presented in this investigation, TMs of the 3rd pass was considered the controlling factor that determines the conditions of stresses at type II boundary. Three levels of TMS of the 3rd pass can be obtained: At TMs ≥ 300 o C, lower compressive stresses are generated from martensitic transformation in the 3rd pass. So, high tensile stresses are residue causing cracking along type II boundary. At TMs ≈ 200 o C, relatively high compressive stresses are generated from martensitic transformation in the 3rd pass. The net stresses which resulted at fusion boundary are tensile but with low magnitude value. These low tensile stresses work as a driving force for type II boundary formation and are not sufficient to cause cracking.
At TMs ≈ 50 o C, high compressive stresses are generated from martensitic transformation in the 3rd pass. In this case, all tensile stresses are compensated. Therefore the driving force for formation of type II boundary is nil hence, type II boundary itself is not created. PWHT is necessary to achieve accepted impact strength (i.e. higher than 27Joule at 0 o C). Thus mechanical