Weld Overlay Repair of Air Compressor High and Low Pressure Cylinders

Literature Overview

This study note addresses the technical challenges and solutions involved in restoring air compressor cylinders through weld overlay repair. Air compressors, particularly in petrochemical and power generation applications, operate under cyclic loading with high-temperature hot gas environments. The high-pressure and low-pressure cylinders are subject to erosion, corrosion, and wear that progressively reduce wall thickness below the minimum allowable limit specified in ASME VIII Div.1. When the remaining thickness falls below the corrosion allowance plus the minimum design thickness, repair by weld overlay becomes a viable alternative to full component replacement, provided the base material is sound and the overlay process is properly qualified.

Core Technical Challenges

The repair of air compressor cylinders presents several interrelated difficulties. The base material is typically a low-alloy steel such as 15CrMo or 12Cr1MoV, which is susceptible to hydrogen-induced cracking during welding due to its higher carbon equivalent and martensitic susceptibility in the heat-affected zone. The overlay material must withstand hot gas erosion at temperatures ranging from 200 to 450 degrees Celsius while maintaining adequate hardness for resistance against particulate wear. The geometric complexity of cylinder bores, including threaded sections, flange connections, and internal ribs, complicates access for welding equipment and inspection.

Base Material Assessment

Before any repair procedure, a thorough assessment of the base material condition is mandatory. The following checks are performed:

• Ultrasonic thickness measurement at multiple axial and circumferential positions
• Dye penetrant testing of the cylinder bore surface to detect micro-cracks
• Hardness survey across the heat-affected zone of previous welds
• Visual inspection for signs of over-temperature exposure or creep damage

If any indication of cracking or unacceptable hardness variation is found, the affected area must be machined away before overlay welding can proceed.

Weld Overlay Process Selection

For air compressor cylinder repair, submerged arc welding (SAW) and gas metal arc welding (GMAW) are the most commonly employed processes. The selection depends on the geometry of the repair area and the required overlay thickness.

For the cylinder bore, which is typically cylindrical with diameters between 300 and 800 mm, a combination of GMAW for the first pass and SAW for subsequent passes is often optimal. The GMAW first pass ensures good fusion with the base metal, while the SAW subsequent passes provide high deposition efficiency for building up the required thickness.

Overlay Material Selection

The overlay material must provide a combination of wear resistance and thermal stability. Commonly used materials include:

• 1Cr13 (410 equivalent) martensitic stainless steel for moderate wear resistance
• 2Cr13 (420 equivalent) for higher hardness requirements
• Stellite 6 (Co-Cr-W alloy) for severe erosion environments
• 17-4PH precipitation-hardening stainless steel for combined corrosion and wear resistance

For hot gas compressor applications, a two-layer approach is frequently adopted: a first layer of 309L or 310S austenitic stainless steel to act as a transition buffer, followed by the functional overlay layer. This reduces the dilution effect from the carbon steel base and minimizes the risk of intermetallic compound formation.

Preheat and Interpass Temperature Control

The carbon equivalent of 15CrMo is approximately 0.42, which places it in the category requiring preheat to prevent cold cracking. The preheat temperature is calculated using the formula:

T_preheat = 200 + 250 × CE - 150 × (1 - S)

where CE is the carbon equivalent and S is the hardenability factor. For 15CrMo with CE = 0.42, this yields approximately 180–220 °C. The interpass temperature must be maintained between 150 and 250 °C to prevent both cold cracking at lower temperatures and excessive grain growth at higher temperatures.

Post-weld heat treatment (PWHT) at 620–650 °C for 2 hours per 25 mm of wall thickness is required to relieve residual stresses and temper any martensitic structures in the heat-affected zone. The cooling rate from PWHT must be controlled to less than 100 °C per hour to prevent re-hardening.

Defect Analysis and Countermeasures

During the repair process, several defect types have been observed and addressed:

Engineering Practice Case

A case study involved the repair of a 12Cr1MoV high-pressure cylinder in a natural gas compressor station. The original wall thickness was 65 mm, and the measured minimum remaining thickness was 48 mm, below the allowable limit of 52 mm. The repair plan specified a total overlay thickness of 8 mm using a two-layer approach: 2 mm of 309L followed by 6 mm of 17-4PH. The preheat temperature was set at 200 °C, interpass at 200 °C, and PWHT at 640 °C for 3 hours. Post-repair ultrasonic testing revealed no defects, and the hardness of the overlay layer measured 380–420 HV, well within the specified range. The cylinder returned to service and has operated reliably for over 18,000 hours without further issues.

Study Insights

The key takeaway from this literature is that weld overlay repair of air compressor cylinders is not merely a matter of depositing material to restore thickness. The success of the repair depends on a systematic approach that integrates base material assessment, process qualification, careful thermal management, and comprehensive post-weld inspection. The transition layer concept is particularly important for dissimilar metal joints, as it effectively manages the dilution gradient and prevents the formation of brittle phases. Engineers should always prioritize process qualification records and maintain detailed weld maps for traceability.