CMT Surfacing of 15-5PH Precipitation Hardening Stainless Steel: Process Parameter Optimization and Property Characterization

Literature Overview

Published in Precision Forming Engineering (2026, Vol. 18, No. 7, pp. 154–165), this study by Lai Shubin, Liu Haoran, and colleagues addresses the lack of experimental data and guidance for CMT (Cold Metal Transfer) surfacing of 15-5PH (UNS S15500) precipitation hardening stainless steel. The research was supported by multiple funding sources including CNOOC Energy Development Equipment Technology Company (202515893886) and the Shaanxi Provincial Natural Science Foundation (2025JC-YBMS-545). The work is directly relevant to offshore oil and gas equipment repair and manufacturing where 15-5PH is widely specified for its excellent combination of strength, toughness, and corrosion resistance.

Process Parameter Investigation

Single-Pass Welding Behavior

The study systematically varied wire feed speed and welding speed to establish the process parameter window:

Parameter Combination	Single-Pass Width	Single-Pass Height	Surface Ripple	Dilution Rate
Low wire feed, constant welding speed	Narrow	Low	High	High
Medium wire feed, constant welding speed	Medium	Peak	Low	Low
High wire feed, constant welding speed	Wide	Decreasing	Increasing	Increasing
Constant wire feed, increasing welding speed	Decreasing	Decreasing	—	—

Key observations:

At constant welding speed, increasing wire feed speed first increases then decreases pass height, indicating an optimal heat input range.
At constant wire feed speed, increasing welding speed uniformly reduces both pass width and height due to decreased heat input.
Both surface ripple and dilution rate exhibit a U-shaped relationship with wire feed speed, with optimal values at intermediate wire feed speeds.

Optimal Parameter Selection

The study identifies wire feed speed of 7 m/min and welding speed of 5 mm/s as providing the best combination of:

Good weld bead profile and surface quality
High hardness suitable for 15-5PH applications
Acceptable tensile properties for structural integrity

Microstructure and Mechanical Properties

Microstructural Characterization

The overlay deposit microstructure consists of:

Primary phase: Equiaxed martensite (dominant)
Secondary phases: Columnar martensite (minor), ferrite, and spherical carbides

The equiaxed martensite morphology is characteristic of CMT's low heat input process, which promotes rapid solidification and suppresses columnar grain growth. This is a significant advantage over conventional SAW or FCAW surfacing where columnar structures are more prevalent.

Mechanical Property Trends

Wire Feed Speed	Grain Size	Tensile Strength	Microhardness	Elongation
Low	Fine	Lower	Fluctuating	Minimal change
Medium	Intermediate	Higher	Fluctuating	Minimal change
High	Coarse	Highest	Fluctuating	Minimal change

The increasing tensile strength with wire feed speed correlates with grain coarsening, which is somewhat counterintuitive but likely reflects the precipitation strengthening response of the 15-5PH matrix to varying cooling rates. The hardness fluctuation rather than monotonic change suggests complex interactions between martensite volume fraction, carbide distribution, and residual austenite content.

Process-Structure-Property Relationships

FMEA Analysis of Critical Parameters

Failure Mode	Effect	Cause	Detection	Prevention
Excessive dilution	Property degradation	High wire feed speed	Spectroscopic analysis of overlay	Limit wire feed speed to ≤7 m/min
Surface porosity	Reduced fatigue life	Excessive welding speed	Visual + penetrant testing	Maintain welding speed ≤5 mm/s
Cracking	Loss of containment	Rapid cooling + high residual stress	RT + MT inspection	Implement post-weld tempering at 480°C
Low toughness	Brittle fracture risk	Excessive martensite without tempering	Charpy impact testing	Post-weld heat treatment mandatory

Implications for 15-5PH Surfacing

The 15-5PH alloy derives its strength from precipitation hardening (Mo-rich and Cu-rich precipitates) that occurs during aging treatment. CMT surfacing introduces unique challenges:

Precipitation state control: The as-welded microstructure must be amenable to subsequent aging to achieve target properties (typically 950–1100 MPa yield strength after H900 or H1025 treatment).
Dilution management: Excessive base metal dilution can reduce the Cr and Mo content below levels needed for precipitation strengthening and corrosion resistance.
Residual stress: The high martensite content in the as-deposited state creates significant residual stresses that must be relieved before aging to prevent cracking.

Engineering Practice Integration

For offshore platform repair applications, this study provides the following practical guidance:

Parameter selection: Use 7 m/min wire feed and 5 mm/s travel speed as starting parameters for 15-5PH CMT surfacing.
Post-weld treatment: Implement stress relief at 480°C for 2 hours followed by aging at 480°C (H900) or 510°C (H1025) to achieve full precipitation hardening response.
Multi-pass strategy: For thick overlays (>3 mm), use the identified parameter window for each pass while maintaining interpass temperature between 150–200°C to manage thermal cycling.
Inspection protocol: Perform 100% visual and penetrant testing of all overlay surfaces; supplement with hardness mapping and ultrasonic examination for critical applications.

Study Insights and Conclusions

This research fills an important gap in the available literature on CMT surfacing of precipitation hardening stainless steels. The systematic approach of establishing the process window first and then investigating microstructure-property relationships provides a methodology that can be adapted to other alloy systems. The finding that elongation shows minimal variation with process parameters suggests that the CMT process produces relatively consistent ductility regardless of parameter variations within the identified window, which is advantageous for quality control in production environments.

The study's practical value lies in providing quantitative parameter guidance that was previously unavailable for 15-5PH CMT surfacing. Engineers working on subsea equipment repair, marine structures, and chemical processing equipment can apply these findings with confidence, subject to appropriate post-weld heat treatment to achieve the full precipitation hardening response of the 15-5PH overlay.