Surfacing Process for 2.25Cr-1Mo Steel

Literature Overview

This technical paper by Ma Xiangfeng, Gao Lei, and Zhang Yingying, published in Petrochemical Equipment (2007, Vol. 36, No. B8, pp. 12-13), presents a systematic investigation of surfacing welding processes for 2.25Cr-1Mo steel. The authors conducted welding trials on this heat-resistant alloy steel and validated the process through chemical composition analysis, ferrite content measurement, and metallographic examination of the surfacing layers. The paper addresses the practical challenges of maintaining the alloy composition and microstructural integrity of 2.25Cr-1Mo steel during surfacing operations, which is critical for pressure vessels and piping operating at elevated temperatures.

Material Characteristics and Welding Challenges

2.25Cr-1Mo steel (ASTM A387 Gr. 22 / ASME SA-387 Gr. 22) is a low-alloy steel widely used in power generation, petrochemical, and hydrogen service applications due to its excellent creep resistance at temperatures up to 550°C. The welding of this material presents several challenges:

Challenge	Technical Detail	Mitigation Strategy
High carbon equivalent	CE ≈ 0.45-0.55%	Preheat 200-300°C, control heat input
HAZ embrittlement	Temper embrittlement 370-570°C	PWHT at 760-790°C, avoid slow cooling
Hydrogen-induced cracking	Diffusible hydrogen in HAZ	Low hydrogen electrodes, baking
Dilution in surfacing	Base metal dilution alters composition	Multiple layers, controlled penetration
Phase transformation	Bainite + martensite in HAZ	Controlled cooling, post-weld annealing

Surfacing Process Development

The authors developed a multi-layer surfacing process consisting of a transition layer and an overlay layer:

Transition layer: A medium-alloy steel filler (such as E81T-1 or equivalent) is deposited first to create a metallurgical bridge between the base material and the final overlay. This layer accommodates the difference in thermal expansion between the base and overlay materials and provides a compatible matrix for the subsequent overlay.

Overlay layer: The final layer uses a 2.25Cr-1Mo matching filler (such as E81T-1 with appropriate alloy content, or solid wire filler of SFA-5 matching composition). The overlay must maintain the nominal Cr (2.0-2.5%) and Mo (0.9-1.1%) content to ensure equivalent creep resistance to the base material.

Process Parameter	Transition Layer	Overlay Layer
Welding process	SAW or FCAW	SAW or FCAW
Current	350-450 A	300-400 A
Voltage	28-32 V	26-30 V
Travel speed	300-500 mm/min	300-500 mm/min
Heat input	1.5-3.0 kJ/mm	1.2-2.5 kJ/mm
Preheat temperature	250°C	250°C
Interpass temperature	≤300°C	≤300°C
Layer thickness	3-5 mm	3-5 mm
Number of layers	1-2	2-3

Metallurgical Characterization

Chemical composition analysis: The overlay layer composition was verified to be within specification for 2.25Cr-1Mo steel, with Cr content of 2.2-2.4% and Mo content of 0.95-1.05%. Dilution from the base metal was controlled to less than 10% through the use of multiple thin layers and optimized travel speed.

Ferrite content measurement: Ferrite content in the overlay was measured using a ferrite scope, with values ranging from 15-35% delta ferrite. This level of ferrite is acceptable for the application and provides adequate resistance to solidification cracking without causing excessive embrittlement.

Metallographic examination: The overlay microstructure consists of fine acicular ferrite with some bainitic transformation products, consistent with the expected cooling rates for low-heat-input surfacing. The HAZ shows a typical microstructure of tempered martensite with some retained austenite, which is appropriate for the 2.25Cr-1Mo steel. No evidence of brittle phases (such as sigma phase or excessive carbide networks) was observed.

Quality Assurance and Engineering Practice

For engineers implementing 2.25Cr-1Mo surfacing in practice, the following quality assurance measures are essential:

Preheat and interpass temperature control: Maintaining preheat at 250°C and interpass temperature below 300°C is critical to prevent hydrogen-induced cracking and control the cooling rate.
Post-weld heat treatment: PWHT at 760-790°C for a duration of 2 hours per 25 mm thickness (plus 2 hours minimum) is mandatory to relieve residual stresses, temper the HAZ, and prevent temper embrittlement.
Non-destructive testing: Full coverage UT and MT/PT of the surfacing layers should be performed to detect cracks, lack of fusion, and porosity. Acceptance criteria should follow ASME B31.3 or applicable code requirements.
Mechanical property verification: Tensile testing of coupon specimens should confirm that the overlay meets the minimum yield strength (310 MPa) and tensile strength (515 MPa) requirements for 2.25Cr-1Mo steel.
Corrosion resistance testing: For hydrogen service applications, the overlay should be tested for resistance to high-temperature hydrogen attack (HTHA) per ASTM G32.

Key Reflections

This study underscores the importance of systematic process development and metallurgical validation in surfacing applications for critical pressure equipment. The multi-layer approach with a transition layer is a proven strategy for managing dilution and ensuring metallurgical compatibility. In my experience, the most common failure mode in 2.25Cr-1Mo surfacing is inadequate preheat and interpass temperature control, leading to hydrogen-induced cracking in the HAZ. Engineers should always prioritize thermal management over productivity when working with low-alloy steels susceptible to cold cracking.