Welding of Clad Carbon Steel Coated By Nickel Base Alloy
Mohamed Morsi Mohamed Farag;
Abstract
1. Cracks occurred in the FCC fusion zone between the FCC 2nd pass (nickel base) and 3rd pass (carbon steel) and not in martensitic zone. Thus the martensitic zone is not the reason for failure
2. Cracking occurred even when hydrogen was removed by baking at 280 oC indicating that residual stresses resulting from welding are the controlling mechanism for failure.
3. Reducing residual stresses by increasing preheat and interpass temperatures did not prevent cracking. Thus nonconventional techniques are required. In this technique, martensitic transformation which occurred in the 3rd pass was used to induce compressive stresses on fusion boundary and to prevent cracking along type II boundary.
4. 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:
a. At TMs ≥ 300oC, lower compressive stresses are generated from martensitic transformation in the 3rd pass. So high tensile stresses are still present causing cracking along type II boundary.
b. At TMs ≈ 200oC, relatively high compressive stresses are generated from martensitic transformation in the 3rd pass. The net stresses obtained at the 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.
c. At TMs ≈ 50oC, 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.
5. TMs can be controlled by chemical composition and by grain size. Based on this fact; two methods were used to reduce TMs of the third pass. The first one was described as "Dilution Method" and the second one was described as "Grain Refining Method".
6. PWHT is necessary to achieve accepted impact strength (i.e. higher than 27Joule at 0oC). Thus mechanical properties are accepted only when cracks are inhabited and PWHT is applied.
7. Welding technique which used AWS5.11 ENi-1 to weld the third pass and AWS E7018 to weld the forth pass, improved the impact strength without a need for PWHT. This improvement was attributed to three reasons.
a. The martensite which resulted from weld carbon steel on AWS 5.11 was iron nickel martensite.
b. High nickel content prevented any expected carbon migration or formation of carbides during welding operation via reducing chromium content within this interfacial region.
c. High nickel content moderated the deleterious effect of macrosegregation islands or peninsulas which appeared near and approximately parallel to fusion boundary
8. Excellent cost saving could be achieved by using AWS5.11 ENi-1, where carbon steel filler metal was used and PWHT was avoided.
2. Cracking occurred even when hydrogen was removed by baking at 280 oC indicating that residual stresses resulting from welding are the controlling mechanism for failure.
3. Reducing residual stresses by increasing preheat and interpass temperatures did not prevent cracking. Thus nonconventional techniques are required. In this technique, martensitic transformation which occurred in the 3rd pass was used to induce compressive stresses on fusion boundary and to prevent cracking along type II boundary.
4. 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:
a. At TMs ≥ 300oC, lower compressive stresses are generated from martensitic transformation in the 3rd pass. So high tensile stresses are still present causing cracking along type II boundary.
b. At TMs ≈ 200oC, relatively high compressive stresses are generated from martensitic transformation in the 3rd pass. The net stresses obtained at the 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.
c. At TMs ≈ 50oC, 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.
5. TMs can be controlled by chemical composition and by grain size. Based on this fact; two methods were used to reduce TMs of the third pass. The first one was described as "Dilution Method" and the second one was described as "Grain Refining Method".
6. PWHT is necessary to achieve accepted impact strength (i.e. higher than 27Joule at 0oC). Thus mechanical properties are accepted only when cracks are inhabited and PWHT is applied.
7. Welding technique which used AWS5.11 ENi-1 to weld the third pass and AWS E7018 to weld the forth pass, improved the impact strength without a need for PWHT. This improvement was attributed to three reasons.
a. The martensite which resulted from weld carbon steel on AWS 5.11 was iron nickel martensite.
b. High nickel content prevented any expected carbon migration or formation of carbides during welding operation via reducing chromium content within this interfacial region.
c. High nickel content moderated the deleterious effect of macrosegregation islands or peninsulas which appeared near and approximately parallel to fusion boundary
8. Excellent cost saving could be achieved by using AWS5.11 ENi-1, where carbon steel filler metal was used and PWHT was avoided.
Other data
| Title | Welding of Clad Carbon Steel Coated By Nickel Base Alloy | Other Titles | لحام الصلب الكربونى المطلى بطبقة من سبيكة النيكل | Authors | Mohamed Morsi Mohamed Farag | Issue Date | 2015 |
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