CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Temperature on CMT Cladding HAZ Microstructure and Properties of 40CrNiMo Steel
Literature Overview
This research paper, published in 2021 in the journal Metal Heat Treatment by Wang Jingbo, Lian Mingyang, Fu Yadi, Liu Shengxin, Huang Zhiquan, and Chen Yong from Zhengzhou University and Zhengzhou Machinery Research Institute, investigates the effect of tempering temperature on the microstructure and properties of the heat-affected zone (HAZ) in cold metal transfer (CMT) cladding of 40CrNiMo quenched and tempered steel. The study is supported by the Zhengzhou Municipal Major Science and Technology Innovation Special Project (188PCXZX784), reflecting the regional emphasis on advancing welding and materials technology. CMT welding is a variant of gas metal arc welding that uses a very low heat input, which is advantageous for cladding applications where dilution and HAZ degradation must be minimized.
Core Technical Analysis
CMT Cladding Process Characteristics
Cold Metal Transfer (CMT) welding is characterized by the use of a pulsed current to control the wire feeding and arc length, resulting in very low heat input and minimal spatter. The low heat input is particularly beneficial for cladding operations on pre-hardened or pre-tempered substrates, as it reduces the extent of thermal damage to the base metal and minimizes the zone of microstructural change.
| CMT Process Parameter | Typical Value | Effect on HAZ |
|---|---|---|
| Peak current | 100-200 A | Controls penetration and dilution |
| Background current | 20-50 A | Maintains arc stability |
| Wire feed speed | 3-8 m/min | Controls deposition rate |
| Travel speed | 200-500 mm/min | Controls heat input per unit length |
| Shielding gas | Pure Ar or Ar + CO2 | Controls arc characteristics and bead profile |
| Heat input | 0.3-0.8 kJ/mm | Low heat input minimizes HAZ |
The low heat input of CMT welding results in a narrow HAZ with a rapid cooling rate, which can lead to the formation of hard, brittle phases such as martensite and bainite in the HAZ of 40CrNiMo steel. The tempering treatment is therefore essential to restore the toughness and ductility of the HAZ while maintaining the strength of the cladding layer.
40CrNiMo Steel Base Material
40CrNiMo is a medium-carbon alloy steel that is commonly used for high-strength applications requiring good toughness and wear resistance. The steel is typically supplied in the quenched and tempered condition, with a typical hardness of HRC 35-42 and a tensile strength of 900-1100 MPa. The microstructure consists of tempered martensite with dispersed carbides, providing a good balance of strength and toughness.
The tempering treatment of the cladded assembly is critical to achieving the desired properties in the HAZ. The tempering temperature must be carefully selected to avoid excessive softening of the base metal while adequately tempering the hard, brittle phases formed during welding. The research investigates tempering temperatures ranging from 450 °C to 650 °C, with holding times of 2 hours.
HAZ Microstructure Evolution
The HAZ microstructure of 40CrNiMo steel after CMT cladding is characterized by a gradient of microstructural phases from the fusion boundary outward into the base metal. The phases present include martensite, bainite, and tempered martensite, depending on the peak temperature reached during welding and the subsequent cooling rate.
| HAZ Region | Peak Temperature | As-Welded Microstructure | After 550 °C Tempering |
|---|---|---|---|
| Fusion boundary | >1300 °C | Fine martensite | Tempered martensite |
| Coarse grain zone | 1100-1300 °C | Coarse martensite | Tempered martensite |
| Partial transformation zone | 900-1100 °C | Mixed martensite/bainite | Tempered martensite/bainite |
| Base metal | <900 °C | Tempered martensite | Tempered martensite |
The tempering treatment transforms the hard, brittle martensite and bainite phases in the HAZ into tempered martensite, which has improved toughness and ductility while maintaining adequate strength. The tempering temperature of 550 °C is identified as the optimal value for 40CrNiMo steel, as it provides the best balance between HAZ toughness and base metal strength retention.
Mechanical Properties
The mechanical properties of the HAZ are evaluated through hardness testing, tensile testing, and impact testing. The hardness profile across the HAZ shows a peak near the fusion boundary, which decreases with distance into the base metal. After tempering, the hardness peak is reduced, and the hardness profile becomes more uniform.
| Condition | HAZ Hardness (HV) | Base Metal Hardness (HV) | Impact Energy (J) |
|---|---|---|---|
| As-welded | 450-500 | 380-400 | 15-25 |
| 450 °C tempered | 400-450 | 370-390 | 30-45 |
| 550 °C tempered | 350-400 | 360-380 | 50-70 |
| 650 °C tempered | 300-350 | 340-360 | 60-80 |
The results demonstrate that tempering at 550 °C provides the optimal balance of HAZ toughness and base metal strength, with impact energy values exceeding 50 J and hardness values remaining within acceptable limits for the intended application.
Engineering Practice Integration
The CMT cladding process, combined with appropriate tempering treatment, offers a viable solution for the repair and enhancement of high-strength alloy steel components. The low heat input of CMT welding minimizes the extent of thermal damage to the base metal, while the tempering treatment restores the toughness of the HAZ without excessive softening of the base material. This combination is particularly advantageous for the repair of components such as shafts, gears, and pressure vessel components that require both wear resistance and toughness.
The research provides guidance for the selection of tempering temperature based on the specific requirements of the application. For applications requiring maximum toughness, a tempering temperature of 550-600 °C is recommended, while for applications requiring maximum strength, a lower tempering temperature of 450-500 °C may be appropriate. The key is to understand the trade-offs between toughness and strength and to select the tempering temperature that provides the best balance for the intended service conditions.
Study Reflections
This research provides valuable insights into the interaction between CMT cladding and tempering treatment for high-strength alloy steels. The systematic investigation of tempering temperature effects on HAZ microstructure and properties offers a practical framework for the optimization of cladding repair processes. The emphasis on the importance of post-weld heat treatment in achieving acceptable HAZ properties highlights a critical aspect of cladding technology that is often overlooked in practice. The key insight from this research is that the CMT process, with its low heat input, provides an excellent foundation for cladding high-strength steels, but that the tempering treatment is essential to achieve the desired balance of properties in the HAZ. This understanding is essential for the successful application of CMT cladding to critical components in the power generation, petrochemical, and heavy equipment industries.