Wear-Resistant Overlay Welding on Large Gas Cylinder Press Plunger Surfaces

Literature Overview

This paper by Zhang Junlin, Wang Junlin, and Chen Zhuohua from Taiyuan Heavy Machinery Factory, published in the journal Welding in 1980, addresses the challenge of extending the service life of large-diameter plungers used in high-pressure gas cylinder presses. The plungers, with diameters ranging from 200 mm to 1000 mm, are subjected to severe wear during the forming of gas cylinder end caps. The authors describe their experimental investigation into the effect of welding parameters on the surface hardness and wear resistance of a 2Cr13 overlay layer, and they document the successful application of the overlay welding process to restore or enhance the wear resistance of these critical components.

Technical Background and Wear Mechanism

The plungers in high-pressure gas cylinder presses are subjected to a combination of wear mechanisms, including abrasive wear from the interaction with the forming die, adhesive wear from the high contact pressure, and erosion wear from the high-velocity deformation of the workpiece material. The base material of the plungers is typically a low-carbon steel or a medium-carbon steel, which has adequate strength for the application but insufficient surface hardness to resist the severe wear conditions. The overlay welding process provides a means of introducing a hard, wear-resistant surface layer without the need to replace the entire plunger.

The 2Cr13 martensitic stainless steel was selected as the overlay material because of its favorable combination of hardness, toughness, and corrosion resistance. When properly heat-treated, 2Cr13 can achieve a hardness of 48-54 HRC, which is significantly higher than the base material and provides adequate resistance to the wear mechanisms encountered in the application. The martensitic microstructure of 2Cr13, characterized by lenticular martensite and retained austenite, provides a good balance of hardness and toughness, reducing the risk of spalling or cracking under cyclic loading.

Welding Parameter Optimization

The authors conducted a systematic study of the welding parameters, with particular emphasis on the welding current, which is the primary factor influencing the heat input and the dilution ratio. The dilution ratio, defined as the volume fraction of base metal in the weld metal, directly affects the final hardness of the overlay layer. A high dilution ratio results in a weld metal that is closer to the base metal composition and therefore lower in hardness, while a low dilution ratio produces a weld metal that is closer to the electrode composition and therefore higher in hardness.

The results indicate that the optimal welding current is in the range of 180-200 A, which provides a dilution ratio of 25-30% and a surface hardness of 46-48 HRC. At lower currents, the dilution ratio is too high, resulting in a softer overlay layer. At higher currents, the heat input is excessive, leading to grain coarsening and a reduction in hardness due to the formation of retained austenite and coarse carbides.

The welding process used was SMAW (Shielded Metal Arc Welding) with a 2Cr13 electrode. The electrode was preheated to 300-350°C to ensure low hydrogen content and to prevent cold cracking. The welding was performed in multiple passes, with each pass being carefully planned to minimize the heat input per pass and to ensure uniform coverage of the plunger surface. The interpass temperature was maintained below 200°C to prevent excessive grain growth in the previous pass.

Overlay Layer Microstructure and Properties

The microstructure of the 2Cr13 overlay layer was examined using optical metallography and scanning electron microscopy. The overlay layer exhibited a martensitic microstructure with lenticular martensite plates and a minor amount of retained austenite. The carbide phase was present in the form of fine M23C6 and MC carbides, which contributed to the hardness of the overlay layer. The grain size of the overlay layer was finer than that of the base metal, which is beneficial for the mechanical properties.

The hardness profile across the overlay layer showed a gradient from the surface to the fusion boundary. The surface hardness was 46-48 HRC, decreasing to 40-42 HRC at the fusion boundary due to the dilution effect. This gradient is acceptable for the application because the surface is the primary region subject to wear, and the lower hardness at the fusion boundary provides a transition to the tougher base metal, reducing the risk of cracking.

The wear resistance of the overlay layer was evaluated using a pin-on-disk wear test. The results showed that the 2Cr13 overlay layer exhibited a wear rate that was 3-4 times lower than that of the base material under the same test conditions. This improvement in wear resistance translates directly to an extension of the plunger service life, reducing the frequency of maintenance and the overall cost of operation.

Engineering Practice Integration

The application of overlay welding to plunger surfaces is a common practice in the heavy machinery industry, but the successful implementation requires careful attention to several practical aspects. First, the surface preparation of the plunger is critical. The surface must be machined to a smooth finish, free of cracks, porosity, and scale, to ensure good fusion and adhesion of the overlay layer. Any existing cracks must be repaired before the overlay welding is performed.

Second, the welding sequence must be planned to minimize distortion. Large-diameter plungers are susceptible to warping due to the asymmetric heat input from the welding process. A symmetric welding sequence, where the weld is deposited in alternating directions around the circumference, can help to distribute the stresses more uniformly. In some cases, a backing plate or a cooling plate may be used to control the heat flow and reduce distortion.

Third, the post-weld heat treatment is essential to achieve the desired hardness and to relieve the residual stresses. The overlay layer is typically hardened by austenitizing at 1000-1050°C followed by oil quenching and tempering at 200-300°C. The tempering temperature is selected to achieve the desired hardness while maintaining adequate toughness. The post-weld heat treatment must be performed carefully to avoid cracking due to the thermal stresses induced by the differential expansion of the overlay layer and the base metal.

Key Questions and Reflections

One question that arises from this paper is the long-term performance of the overlay layer under cyclic loading. The overlay layer is subjected to repeated contact pressure and friction during the forming process, which can lead to fatigue cracking at the fusion boundary. The toughness of the fusion boundary is critical, and the dilution ratio must be controlled to ensure that the fusion boundary has adequate toughness to resist crack initiation and propagation. In my experience, the fusion boundary is the weakest link in the overlay welding system, and the welding procedure must be designed to produce a fusion boundary with a fine microstructure and low residual stress.

Another reflection is that the paper, written in 1980, predates the widespread use of advanced welding processes such as plasma arc welding and laser welding for overlay applications. These processes offer superior control over the heat input and the dilution ratio, which can result in overlay layers with higher hardness and better wear resistance. However, the fundamental principles of overlay welding described in the paper remain valid, and the approach to parameter optimization is still relevant today.

Study Insights and Implications

This paper provides a practical and thorough investigation of the overlay welding process for wear-resistant applications. The systematic approach to parameter optimization, combined with detailed microstructural analysis, demonstrates the importance of understanding the relationship between welding parameters, microstructure, and properties. The paper is particularly valuable for engineers working in the heavy machinery industry, where the extension of component life through overlay welding is a common requirement. The key insight is that the welding parameters must be optimized not only for the overlay layer properties but also for the fusion boundary properties, as the fusion boundary is the critical region that determines the overall performance of the overlay weld. The paper reinforces the principle that overlay welding is a process that requires careful planning, execution, and verification to achieve the desired results.