Manual Arc Welding Overlay Repair of 16MnR Pressure Vessel Heads

Literature Overview

This 2003 technical paper by Jia Hui, Dai Zhongming, and Ma Jingjun from the Maintenance Branch of Shanxi Aluminum Factory, published in Shanghai Chemical Engineering, documents the manual arc welding (SMAW) overlay repair of 16MnR pressure vessel heads. The case study addresses a practical maintenance scenario where a pressure vessel head required overlay repair due to localized corrosion or mechanical damage. The 16MnR material (equivalent to SA-516 Gr.70) is a widely used low-alloy steel for pressure vessels in the petrochemical and chemical industries.

Base Material and Repair Requirements

16MnR is a low-alloy structural steel with the following key properties:

Property	Value	Unit
Yield strength (Rm)	≥345	MPa
Tensile strength (Rm)	490-630	MPa
Elongation (A)	≥21	%
Impact energy (-20°C)	≥34	J
Carbon equivalent (Ceq)	0.38-0.42	%
Hardness (HB)	140-180	-
Temperature range	-20 to +425	°C

The repair requirements for 16MnR pressure vessel heads include:

Restoration of wall thickness to minimum design thickness
Maintenance of mechanical properties (tensile, impact, hardness)
Absence of cracks, porosity, and lack of fusion
Compliance with applicable codes (GB/T 150, NB/T 47002)
Hydrostatic test pressure compliance

Welding Process and Parameters

The manual arc welding (SMAW) process was selected for the overlay repair due to its flexibility, portability, and suitability for complex geometries. The following welding parameters were employed:

Parameter	Value	Unit
Electrode type	E5016 (R316)	-
Electrode diameter	3.2 - 4.0	mm
Welding current	120 - 180	A
Arc voltage	22 - 28	V
Travel speed	50 - 80	mm/min
Preheat temperature	100 - 150	°C
Interpass temperature	≤200	°C
Post-weld heat treatment	580-620°C, 2h	-
Overlay thickness	3-6 mm	-
Number of passes	2-3	-

Process Control and Quality Assurance

The repair procedure follows a systematic approach:

Surface preparation: The damaged area is ground back to sound material, creating a smooth transition with a radius of at least 2 mm. The area is cleaned to remove all contaminants.
Preheating: The entire repair area is preheated to 100-150°C using induction heating or oxy-fuel torches. This reduces the cooling rate and minimizes the risk of cold cracking.
Welding execution: The overlay weld is deposited in multiple passes, starting from the center of the repair area and working outward. The welding direction is maintained to minimize residual stress accumulation.
Post-weld heat treatment (PWHT): The entire vessel head is subjected to PWHT at 580-620°C for a minimum of 2 hours. This relieves residual stresses and improves the microstructure of the HAZ and overlay layer.
Non-destructive testing (NDT): The repair area is inspected using magnetic particle testing (MT) and ultrasonic testing (UT) to verify the absence of surface and volumetric defects.

Defect Analysis and Countermeasures

Defect	Cause	Detection Method	Countermeasure
Cold cracking	High Ceq, low preheat	MT, PT	Increase preheat to 150°C; use low-hydrogen electrodes
Hot cracking	Sulfur segregation, low Mn content	MT, UT	Control electrode composition; avoid high sulfur base metal
Porosity	Moisture in electrodes, surface contamination	RT, UT	Bake electrodes at 350-400°C; clean surface thoroughly
Lack of fusion	Excessive travel speed, low current	UT, MT	Optimize welding parameters; ensure proper root preparation
Hardness exceedance	Rapid cooling, martensitic transformation	Hardness test	Increase preheat; apply PWHT

Engineering Practice Insights

The repair of 16MnR pressure vessel heads using SMAW overlay presents several practical challenges. The curvature of the vessel head complicates welding access and torch positioning. The operator must maintain consistent arc length and travel speed despite the changing geometry. Additionally, the thick section of the vessel head (typically 20-50 mm) creates a significant thermal mass, which affects the cooling rate and, consequently, the microstructure of the weld metal.

The selection of E5016 electrodes is critical. These low-hydrogen electrodes provide adequate strength and toughness while minimizing the risk of hydrogen-induced cracking. The electrodes must be properly stored and baked before use to maintain low hydrogen content. The welding procedure specification (WPS) must be qualified in accordance with NB/T 47014 to ensure the process produces acceptable weld quality.

A key consideration in the repair of pressure vessel heads is the impact of the repair on the overall structural integrity of the component. The overlay weld introduces residual stresses that can affect the stress distribution in the vessel head. The post-weld heat treatment is essential to relieve these stresses and restore the mechanical properties of the HAZ. The PWHT must be performed on the entire component, not just the repair area, to ensure uniform stress relief.

The case study demonstrates that manual arc welding overlay repair is a viable and cost-effective approach for localized damage repair on 16MnR pressure vessel heads. However, it requires careful process control, qualified welders, and thorough quality assurance to ensure the repair meets code requirements and maintains the long-term reliability of the pressure vessel.

Study Insights and Implications

This practical case study provides valuable insights into the repair of low-alloy steel pressure vessel components using manual arc welding overlay techniques. The systematic approach to repair, including proper surface preparation, preheating, welding parameter control, and post-weld heat treatment, is essential for achieving a reliable repair. The experience gained from this case can be applied to similar repairs in the petrochemical and chemical industries, where 16MnR pressure vessels are widely used. The key takeaway is that even simple repair procedures require rigorous quality control to ensure the integrity and safety of pressure-containing equipment.