Zhuojin Pipe Fitting Co., LtdPlasma Surfacing Hardening of Wear-Resistant Cam
Literature Overview
The paper by Liu Liansheng and Wu Shaodan, published in Mining Machinery (Vol. 24, No. 3, 1996, pp. 37-38), reports on the application of plasma arc surfacing technology to harden and improve the wear resistance of cams used in mining machinery. The authors from Ningxia Dawukou Mining Machinery Factory address a practical engineering problem: cams in mining equipment undergo severe sliding and rolling contact wear, leading to frequent replacement and production downtime. The study explores plasma surfacing as a surface engineering solution to extend cam service life.
Core Technical Content
Plasma arc surfacing, classified under TG455 (surfacing welding), involves the use of a transferred or non-transferred plasma arc to deposit a hard alloy layer onto the substrate surface. Unlike conventional surfacing with submerged arc or shielded metal arc processes, plasma surfacing offers several distinct advantages for cam hardening:
- High energy density: The plasma arc provides concentrated heat input, enabling rapid melting and solidification of the surfacing alloy.
- Controlled dilution: The narrow arc column allows precise control of dilution between the surfacing layer and the base material, which is critical for maintaining the hardness and microstructure of the deposited layer.
- Reduced distortion: Lower overall heat input compared to conventional surfacing methods minimizes residual stress and geometric distortion of the cam profile.
The typical process parameters for plasma surfacing of wear-resistant cams include arc current in the range of 80-200 A, arc voltage of 20-35 V, travel speed of 100-400 mm/min, and plasma gas flow rate (usually argon) of 2-5 L/min. The surfacing alloy typically contains high-carbon chromium or chromium-cob-tungsten compositions designed to achieve surface hardness above 50 HRC after appropriate post-weld heat treatment.
Process Analysis and Engineering Considerations
Pre-Weld Preparation
The base material of the cam is typically medium carbon alloy steel (such as 40Cr or 45 steel), which must be machined to final profile dimensions before surfacing. Surface preparation involves grinding the contact surface to Ra ≤ 3.2 μm to ensure good metallurgical bonding. A flux or consumable electrode (such as D257 or D267 type plasma surfacing electrodes) is used to deposit the hard alloy layer.
Key Process Variables
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Arc Current | 80-200 A | Higher current increases dilution and layer thickness |
| Travel Speed | 100-400 mm/min | Higher speed reduces heat input and dilution |
| Arc Voltage | 20-35 V | Controls arc length and stability |
| Plasma Gas Flow | 2-5 L/min | Affects arc stability and shielding effectiveness |
| Preheat Temperature | 150-300°C | Reduces cracking tendency in high-carbon layers |
Post-Weld Treatment
After surfacing, the cam typically undergoes a quenching and tempering cycle. The quenching medium is usually oil or air, depending on the alloy composition. Tempering at 500-600°C relieves residual stresses while maintaining surface hardness. The final surface hardness should exceed 50 HRC to provide adequate wear resistance against the follower or tappet.
Common Defects and Countermeasures
During plasma surfacing of cams, several defects may occur:
- Cracking: High carbon content in the surfacing alloy can lead to hot or cold cracking. Countermeasures include preheating, controlling interpass temperature, and using low-hydrogen consumables.
- Porosity: Inadequate shielding or surface contamination can introduce porosity. Maintaining proper gas flow and cleaning the substrate before surfacing are essential.
- Excessive dilution: If travel speed is too low or arc current too high, excessive dilution reduces the hardness of the surfacing layer. Process parameter optimization through trial runs is necessary.
- Profile distortion: Even with lower heat input, cumulative distortion over multiple passes can affect cam geometry. A machining allowance of 1-2 mm should be provided, and the final profile is achieved by grinding after surfacing.
Engineering Practice Insights
From a practical standpoint, the application of plasma surfacing to cam hardening represents a cost-effective maintenance strategy. Instead of replacing the entire cam, surfacing allows repeated hardening of the worn surface, significantly extending component life. In mining applications where cams operate under heavy loads with abrasive contact, the plasma-surfaced layer can increase service life by 3-5 times compared to the original unhardened surface.
The study also highlights the importance of matching the surfacing alloy to the specific wear mechanism. For abrasive wear (as in mining cam applications), high-carbon chromium alloys with hard carbide particles (Cr7C3, Cr3C2) provide excellent resistance. For adhesive wear, cobalt-based alloys with tungsten carbide particles are more appropriate.
Study Reflections
This 1996 paper, though dated, captures a fundamental principle in surface engineering: the selection of surfacing technology must be matched to both the wear mechanism and the geometric constraints of the component. Cams, with their complex profile geometry, present a particular challenge for surfacing because the deposited layer must maintain dimensional accuracy. Plasma surfacing, with its controllable heat input and narrow bead width, is well-suited for this application.
The paper's emphasis on practical industrial application rather than fundamental research reflects the engineering culture of Chinese mining machinery manufacturers during that era. While modern plasma surfacing processes have advanced significantly (including fully automated systems with computer-controlled parameter optimization), the fundamental principles remain valid. Engineers working on surface hardening of complex-shaped components should still consider plasma surfacing as a viable option, particularly when the component geometry makes other processes (such as HVOF or thermal spray) impractical.
The key takeaway is that surface engineering is not merely about achieving high hardness; it is about achieving the right combination of hardness, toughness, and bond strength under specific operating conditions. The plasma surfacing of cams demonstrates this principle clearly.