Study Note on Stainless Steel Cladding Layer Cracking and Solutions

Problem Statement and Industry Context

This paper, published in 2010 by Gao Chunhua from Daqing Petrochemical Company Ethylene Engineering Command and Xing Fang, Liu Bin, and Han Qing from Daqing Petrochemical Company Machinery Factory, addresses a persistent and critical problem in the weld overlay industry: cracking in stainless steel cladding layers. Stainless steel cladding is widely used in the petrochemical industry to provide corrosion resistance to carbon steel pressure vessels, heat exchangers, and piping systems. However, cracking in the cladding layer is a common failure mode that can lead to leakage, contamination, and catastrophic equipment failure. The study aims to identify the root causes of cracking and to propose effective countermeasures.

Cracking Mechanisms and Root Cause Analysis

The authors conducted a comprehensive investigation of cracking in stainless steel cladding layers using a combination of macroscopic examination, microscopic analysis, and chemical analysis. The cracking was found to occur in three distinct locations: within the cladding layer (intra-layer cracking), at the interface between the cladding layer and the base metal (interfacial cracking), and in the heat-affected zone of the base metal (HAZ cracking). Each type of cracking has a different mechanism and requires a different approach to prevention.

The root cause analysis revealed that the primary causes of intra-layer cracking were the high sulfur and phosphorus content in the filler metal, which promote the formation of low-melting-point eutectics at the grain boundaries during solidification. The authors found that the sulfur content in some of the filler metals used was as high as 0.04 percent, which is well above the recommended limit of 0.02 percent for stainless steel overlay welding. The interfacial cracking was primarily caused by excessive dilution of the base metal into the cladding layer, which led to the formation of a brittle carbide-rich zone at the interface. The HAZ cracking was attributed to the formation of hard, brittle martensite in the heat-affected zone of the carbon steel base, exacerbated by hydrogen absorption from the welding process.

Countermeasures and Process Optimization

The authors proposed a multi-pronged approach to prevent cracking in stainless steel cladding layers. The first measure was the strict control of the filler metal composition, particularly the sulfur, phosphorus, and carbon content. The authors recommended the use of filler metals with sulfur content below 0.02 percent, phosphorus content below 0.03 percent, and carbon content below 0.03 percent. The second measure was the optimization of the welding parameters to minimize the dilution ratio and the cooling rate. The authors recommended the use of a multi-pass welding strategy with a thin first pass to establish a sound metallurgical bond, followed by subsequent passes with a higher deposition rate.

The third measure was the application of a preheat and interpass temperature control strategy. The authors recommended a preheat temperature of 150–200 °C for carbon steel base metals and an interpass temperature of 100–150 °C to prevent excessive cooling rates and hydrogen-induced cracking. The fourth measure was the use of a post-weld heat treatment to relieve residual stresses and to temper any martensite that may have formed in the heat-affected zone. The recommended heat treatment was 650–700 °C for 2 hours, followed by air cooling.

Case Study and Validation

The authors validated the proposed countermeasures through a case study involving the repair of a cracked stainless steel cladded heat exchanger tube sheet in the Daqing Petrochemical Company. The tube sheet was made of carbon steel with a 3 mm thick 309L stainless steel cladding layer. The original cladding had been applied using a single-pass process with no preheat, resulting in extensive cracking at the interface and within the cladding layer. The repair involved the complete removal of the original cladding, the application of a multi-pass cladding process with the optimized parameters, and the post-weld heat treatment.

The repaired tube sheet was subjected to a series of non-destructive examinations, including ultrasonic testing, magnetic particle inspection, and dye penetrant testing. No indications of cracking were found. The hydrostatic test at 1.5 times the design pressure was also successfully completed without leakage. The tube sheet was returned to service and has been operating without any cracking for over three years.

Summary and Engineering Recommendations

The study provides a comprehensive analysis of cracking mechanisms in stainless steel cladding layers and proposes effective countermeasures that have been validated through practical application. The key takeaway for engineers is that cracking is not an inevitable consequence of stainless steel cladding but rather a result of poor process control and inadequate material selection. By implementing the recommended measures, including strict control of filler metal composition, optimized welding parameters, and appropriate heat treatment, the risk of cracking can be significantly reduced. The study also emphasizes the importance of root cause analysis in identifying the specific type of cracking and applying the appropriate countermeasures. Engineers should not apply a one-size-fits-all approach but should tailor the process to the specific material combination, geometry, and service conditions of each application.