Zhuojin Pipe Fitting Co., LtdManual Welding Overlay Process for Stainless Steel Pipe Inner Wall
Literature Overview
The paper by Sun Yong, published in Oil and Gas Field Surface Engineering in 2008 (Vol. 27, No. 6, p. 68), describes a practical manual arc welding overlay process for the inner wall of stainless steel pipes with dimensions of 70 mm outer diameter, 6 mm wall thickness, and 65 mm length. The work was conducted at the Daqing Petroleum Administration Water Supply Engineering Company, reflecting a real industrial application in oil and gas water supply systems. The paper is classified under TG455 and focuses on the specific challenges of overlay welding on small-diameter thin-walled pipes, where access is limited and distortion control is critical.
Technical Challenges of Small-Diameter Pipe Overlay
Overlay welding on the inner wall of small-diameter pipes presents unique challenges that distinguish it from conventional overlay welding on flat plates or large-diameter vessels. The primary difficulties include limited welding access, restricted electrode angle control, heat concentration in a confined geometry, and the risk of burn-through due to the thin wall thickness. The 70 mm outer diameter and 6 mm wall thickness combination means that the heat input per unit length of pipe circumference is significantly higher than in large-diameter applications, creating a high risk of thermal distortion and interfacial cracking.
The paper identifies two primary failure modes that must be controlled: welding cracks and delamination. Cracks can occur in the weld metal itself due to hydrogen embrittlement, low-temperature cracking, or solidification cracking. Delamination occurs at the weld-to-base metal interface and is driven by hydrogen accumulation, residual stress, and metallurgical incompatibility between the overlay material and the base pipe material.
Process Parameters and Control Measures
The paper describes a process that incorporates preheating and post-weld heat treatment as key control measures. Preheating serves multiple purposes: it reduces the cooling rate at the weld zone, allows hydrogen to escape before solidification, and reduces the thermal stress gradient between the weld and the surrounding base metal. For a pipe of this geometry, a preheat temperature of 150-200 °C is recommended, applied uniformly around the circumference to avoid localized thermal gradients.
| Process Parameter | Specification | Rationale |
|---|---|---|
| Pipe dimensions | 70 mm OD x 6 mm WT x 65 mm | Small diameter, thin wall |
| Welding method | Manual arc welding (SMAW) | Access flexibility in confined space |
| Preheat temperature | 150-200 °C | Reduces cooling rate, relieves residual stress |
| Post-weld heat treatment | Stress relief at 400-500 °C | Eliminates residual tensile stress |
| Electrode type | Low-hydrogen stainless steel electrode | Minimizes hydrogen absorption |
| Current type | DCEN preferred | Deeper penetration, narrower bead |
| Inspection method | Dye penetrant testing (PT) | Surface defect detection |
| Defect acceptance | No cracks or delamination | Zero tolerance for interfacial defects |
The post-weld heat treatment is particularly important for this application. A stress relief treatment at 400-500 °C for a duration proportional to the wall thickness effectively reduces the residual tensile stresses that would otherwise promote hydrogen-induced cracking. The temperature must be carefully controlled to avoid sensitization of the stainless steel, which would reduce corrosion resistance. For austenitic stainless steels, temperatures above 500 °C should be avoided to prevent chromium carbide precipitation at grain boundaries.
Quality Verification
The paper reports that dye penetrant testing (PT) was used to verify the quality of the overlay welds. PT is a surface-sensitive method that can detect cracks, pores, and other discontinuities that are open to the surface. For the application described, PT is an appropriate inspection method because the critical defects are surface-breaking cracks and delamination cracks that extend to the surface. The paper reports that no defects were detected by PT inspection, indicating that the process parameters and control measures were effective.
It is worth noting that PT alone may not be sufficient for all applications. For critical service conditions, supplementary non-destructive testing such as ultrasonic testing (UT) or radiographic testing (RT) may be warranted to detect subsurface defects. However, for the water supply application described in the paper, PT provides an adequate level of quality assurance at a reasonable cost.
Practical Implications and Reflections
This paper, while brief, captures an important aspect of industrial welding practice: the adaptation of standard welding procedures to non-standard geometries. The 70 mm pipe with 6 mm wall thickness is not a common configuration in standard welding procedure qualifications, and the welding engineer must exercise judgment in selecting appropriate parameters. The paper's approach of using preheating and post-weld heat treatment as the primary control measures is pragmatic and effective.
From a modern perspective, the process described could be enhanced by incorporating additional monitoring techniques. Real-time thermocouple monitoring of the weld zone temperature would allow precise control of the cooling rate and interpass temperature. Infrared thermography could be used to monitor the uniformity of preheat application around the pipe circumference. These enhancements would improve process consistency and reduce the risk of defects.
The paper also highlights an important principle in overlay welding: the effectiveness of a welding process depends not only on the welding parameters themselves but also on the thermal treatment before and after welding. Preheating and post-weld heat treatment are not optional extras but integral parts of the welding process for applications where hydrogen cracking and delamination are concerns. This principle applies equally to modern welding operations and should be emphasized in welding procedure specifications.
In conclusion, Sun Yong's paper provides a practical and effective approach to overlay welding on small-diameter stainless steel pipes. The combination of preheating, controlled welding parameters, and post-weld heat treatment successfully controls the two primary failure modes of cracking and delamination. The paper's value lies in its demonstration that standard welding techniques, when properly adapted to the specific geometry and service conditions, can achieve reliable results in challenging industrial applications.