Corrosion-Resistant Overlay Welding of 20MnMo Dished Heads

Literature Overview

The technical paper by Guo Baoping (1992, Welding Journal, No. 4, pp. 24-26) documents the corrosion-resistant overlay welding of 20MnMo dished heads for a carbon black water settling tank operating at 9 MPa design pressure and 300°C design temperature. The base material consists of 20MnMo forged dished heads in quenched and tempered condition conforming to J8755-85 Grade IV specifications, with a 4 mm thick austenitic stainless steel overlay required on the internal surface. This case study represents a classic engineering challenge in pressure vessel manufacturing: combining the mechanical strength of low-alloy steel with the corrosion resistance of austenitic stainless steel through overlay welding.

Base Material Characteristics

The 20MnMo base material properties are critical for overlay welding procedure development:

Property	Specification	Value
Carbon (C)	J8755-85 Grade IV	≤ 0.25%
Manganese (Mn)	J8755-85 Grade IV	0.9-1.2%
Molybdenum (Mo)	J8755-85 Grade IV	0.25-0.35%
Yield Strength	Q&T condition	≥ 345 MPa
Tensile Strength	Q&T condition	≥ 520 MPa
Elongation	Q&T condition	≥ 21%
Impact Energy (20°C)	Q&T condition	≥ 47 J

The carbon equivalent (CE) of 20MnMo is approximately 0.42-0.48%, placing it in the moderate crack susceptibility category that requires careful preheat and interpass temperature control during welding.

Overlay Welding Procedure Development

Filler Metal Selection

For overlaying austenitic stainless steel on low-alloy steel, the filler metal selection must address several metallurgical challenges:

Filler Type	Composition	Application	Dilution Control
309L (ENiCr-3)	23% Cr, 12-14% Ni	First layer (high dilution)	Designed for high dilution
316L (ENiCr-3)	17-19% Cr, 10-14% Ni, 2-3% Mo	Subsequent layers	Lower dilution tolerance
316L powder (SAW)	Similar to 316L wire	Multi-pass buildup	Consistent composition

Welding Process Selection

The 4 mm overlay thickness requires multiple passes, and the process selection must balance deposition efficiency, dilution control, and weld quality:

Process	First Layer	Subsequent Layers	Final Layer
SMAW	E309L	E316L	E316L
SAW (with flux)	—	F309L	F316L
GMAW	ER309L	ER316L	ER316L

Process Parameters

Parameter	First Layer	Subsequent Layers	Rationale
Preheat temperature	150-200°C	150-200°C	Reduce cracking in 20MnMo
Interpass temperature	< 250°C	< 250°C	Control HAZ properties
Arc voltage	22-26 V	24-30 V	Control penetration
Current	100-140 A	200-300 A	Higher deposition for buildup
Travel speed	100-150 mm/min	150-250 mm/min	Match heat input
Weld leg size	2-3 mm	3-4 mm	Controlled overlap

Metallurgical Challenges and Solutions

Dilution Control

The first overlay layer experiences the highest dilution from the base metal, potentially reducing the chromium and nickel content below the threshold for adequate corrosion resistance. The solution involves:

Using a high-nickel, high-chromium first layer filler (309L) to compensate for dilution
Minimizing first layer penetration to reduce base metal contribution
Using subsequent layers with the desired final composition (316L) to dilute the first layer composition

Heat-Affected Zone Considerations

The 20MnMo HAZ experiences the following microstructural changes during overlay welding:

HAZ Region	Microstructure	Mechanical Effect
Fusion zone	Mixed austenite-martensite	Variable hardness
Coarse grain zone	Coarse martensite	Reduced toughness
Fine grain zone	Fine martensite	Moderate hardness
Inter-critical zone	Mixed ferrite-martensite	Balanced properties
Sub-critical zone	Tempered martensite	Minimal change

Residual Stress Management

The overlay welding of stainless steel on low-alloy steel introduces significant residual stresses due to:

Thermal expansion coefficient mismatch (austenitic SS: 17.3 × 10⁻⁶/°C vs. 20MnMo: 12.2 × 10⁻⁶/°C)
Multiple thermal cycles from multi-pass welding
Constraint from the parent metal during cooling

Mitigation Strategy	Effectiveness	Implementation
Controlled preheat	Moderate	150-200°C uniform heating
Interpass temperature control	High	Maintain below 250°C
Post-weld stress relief	High	600°C × 2h (if compatible)
Backing ring or backing weld	Moderate	Reduce拘束
Sequential welding pattern	High	Minimize拘束 stress

Quality Control Requirements

Non-Destructive Testing

NDT Method	Coverage	Acceptance Criteria	Purpose
Visual inspection (VT)	100%	No cracks, undercut, porosity	Surface quality
Magnetic particle testing (MT)	100%	No linear indications	Surface cracks
Radiographic testing (RT)	100%	AWS D1.6 Level II	Volumetric defects
Ultrasonic testing (UT)	100%	No delamination	Bond integrity
Hardness testing	Grid pattern	Within specified range	Dilution verification

Corrosion Testing

For the carbon black water service environment, the following corrosion tests are recommended:

Test Method	Duration	Acceptance Criteria	Standard
Immersion test (service fluid)	720 h	No pitting, no corrosion	ASTM G48
Intergranular corrosion test	24 h	No intergranular attack	ASTM A262 Practice E
Stress corrosion cracking test	1000 h	No cracking	ASTM G48 Practice B
Electrochemical polarization	Continuous	Corrosion rate < 0.05 mm/y	ASTM G5

Engineering Practice Integration

This case study exemplifies the systematic approach required for overlay welding of dissimilar materials in pressure vessel applications. The key engineering considerations include:

Code compliance: The overlay welding procedure must be qualified in accordance with ASME Section IX or equivalent national standards, with specific attention to the dissimilar material weld qualification requirements.
Thermal management: The combination of preheat, interpass temperature control, and post-weld heat treatment must be carefully coordinated to prevent cracking in the base metal while maintaining the corrosion resistance of the overlay.
Geometric considerations: The dished head geometry introduces variable welding positions and restricted access, requiring flexible process parameters and potentially multiple welding setups.
Service environment: The 300°C operating temperature and 9 MPa design pressure impose strict requirements on the overlay's mechanical integrity and corrosion resistance at elevated temperature.

Key Questions and Reflections

The most significant engineering challenge in this application is the long-term integrity of the overlay under combined mechanical and corrosion loading at 300°C. While the as-welded properties may be satisfactory, the following degradation mechanisms must be considered:

Sensitization: Prolonged exposure to 300°C may promote chromium carbide precipitation at grain boundaries, reducing intergranular corrosion resistance
Thermal fatigue: Repeated thermal cycling during service may cause cracking at the overlay-base metal interface
Creep: At 300°C, creep deformation may be significant in the overlay material over extended service periods
Hydrogen-induced cracking: If the service environment contains hydrogen, the overlay may be susceptible to hydrogen embrittlement

Study Insights and Implications

This engineering case study demonstrates the practical application of overlay welding technology for combining mechanical strength and corrosion resistance in pressure vessel components. The successful implementation requires a comprehensive approach that integrates metallurgical understanding, process engineering, and quality assurance. The key insight is that overlay welding is not merely a surface treatment but a structural modification that must be designed, qualified, and maintained as an integral part of the pressure boundary. For engineering practice, the critical lessons include the importance of dilution control through filler metal selection, the necessity of thermal management to prevent base metal cracking, and the requirement for comprehensive quality control including both mechanical and corrosion testing. The 20MnMo to austenitic stainless steel overlay represents a well-established engineering solution that, when properly executed, provides reliable long-term service in corrosive environments at elevated temperatures.