Large-Area Surfacing of Cobalt-Chromium-Tungsten Cemented Carbide

Literature Overview

This paper by Li Falin and Long Benren from Jiangxi Jianglian Energy and Environmental Protection Co., Ltd., published in Welding (Issue 1, 2008), addresses the practical challenges of applying cobalt-chromium-tungsten cemented carbide surfacing over large areas on gasification equipment components. Unlike conventional surfacing applications where overlay thickness is limited to a few millimeters, large-area surfacing requires special process control to maintain metallurgical integrity, dimensional accuracy, and surface quality across extended surfaces. This work is particularly relevant to engineers working on gasification plant components, including coal water slurry pump parts, gasifier internals, and wear plates.

Technical Challenges of Large-Area Surfacing

Large-area surfacing of cemented carbide alloys presents unique challenges compared to conventional surfacing:

Thermal distortion: The high thermal input required for multi-pass surfacing over large areas leads to significant distortion and residual stress accumulation.
Cracking susceptibility: Co-Cr-W alloys have high carbon equivalent and are prone to cracking during solidification and cooling.
Carbide coarsening: Extended heat input causes coarsening of WC and Cr₇C₃ carbides, reducing hardness and wear resistance.
Dimensional control: Maintaining flatness and dimensional tolerance over large areas is difficult without post-weld machining.
Surface quality: Achieving consistent surface finish across large areas requires careful parameter optimization.

Process Parameters and Control Strategy

The following table summarizes typical process parameters for large-area Co-Cr-W cemented carbide surfacing:

Parameter	Typical Range	Control Objective
Welding process	SMAW or plasma arc	High deposition rate with controlled heat input
Electrode/rod diameter	2.5–4.0 mm	Balance deposition rate and heat input
Current	120–200 A (SMAW)	Maintain arc stability and penetration
Travel speed	30–80 mm/min	Control bead width and overlap
Preheat temperature	200–350°C	Reduce cracking susceptibility
Interpass temperature	200–300°C	Prevent excessive cooling between passes
Number of passes	3–8	Build up required thickness gradually
Bead overlap	1/3 to 1/2 bead width	Ensure complete fusion and uniform microstructure

Process Design Methodology

The authors employed a systematic approach to develop the surfacing procedure, which can be summarized using the PDCA cycle:

Plan: Determine the required overlay thickness, surface quality, and performance requirements based on the service environment.
Do: Develop and trial the welding procedure with progressive parameter optimization.
Check: Evaluate weld appearance, hardness distribution, microstructure, and dimensional accuracy.
Act: Refine parameters based on test results and establish the final WPS.

Key process design decisions include:

Welding sequence: For large flat areas, a back-step or zigzag sequence should be used to minimize distortion. For curved surfaces, the sequence should follow the curvature to distribute thermal input evenly.
Layer thickness: Each pass should be limited to 2–3 mm to prevent excessive cooling stress and cracking.
Bevel preparation: A proper bevel or groove preparation ensures adequate dilution control and fusion. For thick overlays, a stepped groove design may be necessary.
Post-weld treatment: Stress relief at 400–500°C for 2–4 hours per inch of thickness is recommended to reduce residual stresses without significantly affecting hardness.

Quality Control Considerations

For large-area surfacing applications, the following quality control measures are essential:

QC Method	Purpose	Acceptance Criteria
Visual inspection (VT)	Surface quality, undercut, porosity	No visible defects, smooth surface
Magnetic particle testing (MT)	Surface and near-surface cracks	No linear indications
Ultrasonic testing (UT)	Lack of fusion, internal porosity	No indications above threshold
Hardness testing	Uniformity and minimum hardness	HV ≥ 800, variation ≤ 15%
Dimensional check	Flatness, thickness uniformity	Within ±0.5 mm tolerance

Engineering Practice Cases

In gasification equipment applications, Co-Cr-W cemented carbide surfacing is commonly applied to:

Coal water slurry pump impellers and wear rings: Subjected to high-velocity slurry impact with abrasive coal particles.
Gasifier wear plates: Exposed to high-temperature corrosive gases with solid particle erosion.
Slurry valve seats: Requiring precise dimensional accuracy and wear resistance.

A typical case involves surfacing a 500 mm × 500 mm wear plate with a 6 mm overlay. The procedure requires 4–5 passes with controlled interpass temperature, followed by stress relief and surface grinding to achieve the final dimension. The total cycle time, including preparation, welding, stress relief, and machining, is approximately 12–16 hours per plate.

Key Questions and Reflections

The paper does not provide detailed information on the microstructural characterization of the large-area surfacing, such as carbide size distribution, grain structure, or residual stress measurements. In practice, these factors are critical for predicting service life. Additionally, the paper does not address the effect of base material properties on the surfacing quality. Different base materials (carbon steel, low-alloy steel, stainless steel) will result in different dilution levels and microstructural transitions at the weld interface.

Study Insights and Practical Recommendations

The most important takeaway from this paper is that large-area surfacing of Co-Cr-W cemented carbide is achievable but requires careful process design and quality control. The key success factors are:

Controlled heat input: Use lower current and higher travel speed to minimize carbide coarsening and cracking.
Proper preheating: Maintain preheat at 200–350°C throughout the welding operation, using induction heating or gas torches for large areas.
Stress relief: Always include a post-weld stress relief step to prevent delayed cracking and distortion.
Surface finishing: Plan for post-weld machining to achieve dimensional accuracy and surface quality requirements.

Engineers should not attempt large-area surfacing without a qualified welding procedure specification developed through systematic trial and testing. The cost of rework due to cracking or distortion far exceeds the cost of proper procedure development.