Corrosion-Resistant Cladding of 20MnMo Heads

Literature Overview

This technical paper, published in 1992 by Guo Baoping from Lanzhou Long March Machinery Factory, addresses the practical challenge of applying corrosion-resistant cladding to 20MnMo steel heads. The 20MnMo steel is a medium-carbon low-alloy steel widely used in pressure vessel fabrication for its good combination of strength and weldability. The paper appeared in the journal "Welding" and represents early Chinese engineering experience in bimetal pressure vessel fabrication.

Technical Background

20MnMo steel contains approximately 0.17–0.23% C, 0.85–1.15% Mn, and 0.25–0.35% Mo. The molybdenum addition provides enhanced strength at elevated temperatures and improved resistance to hydrogen blistering. However, this steel offers limited resistance to corrosive media, necessitating cladding with stainless steel or nickel-based alloys when used in aggressive environments.

Substrate Characteristics

Property	20MnMo Steel	Typical Requirement
Tensile strength	490–630 MPa	≥ 490 MPa
Yield strength	295–355 MPa	≥ 295 MPa
Carbon equivalent	0.35–0.45%	< 0.45%
Preheat temperature	150–250°C	Required for cladding
Heat input range	10–30 kJ/cm	Process dependent

Cladding Process Selection

The paper discusses the application of submerged arc welding (SAW) overlay as the primary method for corrosion-resistant cladding of 20MnMo heads. The selection of overlay material depends on the specific service environment:

Overlay Material Selection Criteria

Service Environment	Recommended Overlay	Standard Reference
Mild corrosive (water, weak acids)	304L stainless steel	GB/T 150
Moderate corrosive (chlorides)	316L stainless steel	GB/T 150
High-temperature oxidizing	321/347 stainless steel	ASME II
Severe corrosive	Inconel 625	ASTM B335

Process Parameters for SAW Overlay

Parameter	First Pass	Subsequent Passes
Current	500–600 A	450–550 A
Voltage	28–32 V	26–30 V
Travel speed	250–350 mm/min	300–400 mm/min
Wire diameter	2.4 mm	2.4 mm
Flux type	Low-hydrogen (HJ431)	Low-hydrogen (HJ431)

Transition Zone Challenges

The metallurgical compatibility between 20MnMo steel and stainless steel overlay presents several challenges:

Carbon segregation: During the first pass, carbon from the substrate can be concentrated at the fusion line, potentially leading to intergranular sensitization of the overlay.
Dilution control: The first pass typically achieves 20–30% substrate dilution, which may compromise the corrosion resistance of the resulting deposit.
Residual stress: The thermal expansion mismatch between carbon steel and stainless steel generates significant residual stresses in the transition zone.

Countermeasures for Transition Zone Issues

Use a transition layer with intermediate composition (e.g., low-carbon stainless steel or Ni-Cr alloy) to reduce carbon segregation.
Apply a higher current on the first pass to achieve deeper substrate penetration and more uniform dilution.
Implement post-weld heat treatment (PWHT) at 620–650°C for stress relief.
Perform intergranular corrosion testing (ASTM A262 Practice E or GB/T 4334) on the transition zone.

Quality Assurance Requirements

For pressure vessel applications, the following inspections are mandatory:

Inspection Method	Acceptance Criteria	Standard
Magnetic particle testing (MT)	No linear indications	JB/T 4730.4
Ultrasonic testing (UT)	No lack of fusion	JB/T 4730.3
Hardness testing	Overlay hardness within specified range	GB/T 231
Bond strength test	≥ 150 MPa	ASTM A263
Intergranular corrosion	No intergranular attack	GB/T 4334

Engineering Practice Reflections

This early paper from 1992 reflects the growing Chinese experience in bimetal pressure vessel fabrication during the 1990s. Several observations are relevant to modern practice:

The paper highlights the importance of preheating 20MnMo steel before cladding, a practice that remains critical today due to the steel's moderate carbon equivalent.
The challenges of dilution control and transition zone integrity discussed in this paper are still encountered in current fabrication work.
The paper's focus on SAW overlay reflects the industrial preference for this method due to its high deposition rate and consistent quality, though PTA and laser cladding have since become competitive alternatives.

The work underscores the principle that successful cladding of low-alloy steel substrates requires careful attention to both process parameters and material selection, particularly in pressure vessel applications where weld integrity is directly linked to safety.