How to reduce the residual stress of roller chain after welding

How to reduce the residual stress of roller chain after welding
In the production and manufacturing process of roller chain, welding is a key process. However, there will often be residual stress in roller chain after welding. If effective measures are not taken to reduce it, it will have many adverse effects on the quality and performance of roller chain, such as reducing its fatigue strength, causing deformation and even fracture, thus affecting the normal use and life of roller chain in various mechanical equipment. Therefore, it is very important to deeply study and master the methods to reduce the residual stress of roller chain welding.

1. Causes of residual stress
During the welding process, the welding part of the roller chain will be subjected to uneven heating and cooling. During welding, the temperature of the weld and the surrounding area rises rapidly, and the metal material expands; and during the cooling process, the metal contraction in these areas is constrained by the surrounding unheated metal, thus generating welding residual stress.
The constraint conditions during welding will also affect the size and distribution of residual stress. If the roller chain is highly constrained during welding, that is, the degree of fixed or restricted deformation is large, then during the cooling process after welding, the residual stress caused by the inability to shrink freely will also increase accordingly.
The factors of the metal material itself cannot be ignored. Different materials have different thermal physical and mechanical properties, which will lead to different thermal expansion, contraction and yield strength of materials during welding, thus affecting the generation of residual stress. For example, some high-strength alloy steels have high yield strength and are prone to generate large residual stress during welding.

2. Methods to reduce residual stress in roller chain welding

(I) Optimize welding process

Reasonably arrange welding sequence: For roller chain welding, welds with large shrinkage should be welded first, and welds with small shrinkage should be welded later. This allows the weld to shrink more freely during welding, reducing the residual stress caused by the restricted shrinkage of the weld. For example, when welding the inner and outer chain plates of a roller chain, the inner chain plate is welded first, and then the outer chain plate is welded after it cools down, so that the weld of the inner chain plate is not restricted too much by the outer chain plate when shrinking.

Use appropriate welding methods and parameters: Different welding methods have different residual stresses on roller chains. For example, gas shielded welding can reduce the heat affected zone to a certain extent compared with some traditional welding methods due to its concentrated arc heat and high thermal efficiency, thereby reducing residual stress. At the same time, it is also crucial to reasonably select parameters such as welding current, voltage, and welding speed. Excessive welding current will lead to excessive weld penetration and excessive heat input, which will cause the weld joint to overheat and increase residual stress; while appropriate welding parameters can make the welding process more stable, reduce welding defects, and thus reduce residual stress.
Control interlayer temperature: When welding roller chains in multiple layers and multiple passes, controlling the interlayer temperature is an effective measure to reduce residual stress. Appropriate interlayer temperature can keep the metal of the weld and heat-affected zone in good plasticity during the welding process, which is conducive to the shrinkage of the weld and the release of stress. Generally, the interlayer temperature should be determined according to the properties of the materials used in the roller chain and the welding process requirements, and the temperature during the welding process should be measured and controlled to ensure that the interlayer temperature is within the appropriate range.
(II) Adopt appropriate welding preheating and post-heating measures
Preheating: Before welding the roller chain, preheating the weldment can effectively reduce welding residual stress. Preheating can reduce the temperature difference of the weld joint and make the temperature distribution of the weldment more uniform during welding, thereby reducing thermal stress caused by temperature gradient. In addition, preheating can also increase the initial temperature of the weldment, reduce the temperature difference between the weld metal and the base material, improve the performance of the welded joint, reduce the generation of welding defects, and thus reduce residual stress. The determination of the preheating temperature should be based on the composition, thickness, welding method and ambient temperature of the roller chain material.
Post-heating: Post-heat treatment after welding, that is, dehydrogenation treatment, is also one of the important means to reduce the residual stress of roller chain welding. Post-heat treatment usually heats the weldment to about 250-350℃ immediately after the welding is completed and cooled to a certain temperature, and then slowly cools after keeping warm for a certain period of time. The main function of post-heating is to accelerate the diffusion and escape of hydrogen atoms in the weld and heat-affected zone, reduce the hydrogen content in the weldment, thereby reducing the possibility of hydrogen-induced stress corrosion cracking, and also help to relieve welding residual stress. Post-heat treatment is particularly important for the welding of some high-strength steels and thick-walled roller chains.
(III) Perform post-weld heat treatment
Overall high-temperature tempering: Place the entire roller chain in a heating furnace, slowly heat it to about 600-700℃, keep it warm for a certain period of time, and then cool it to room temperature with the furnace. This overall high-temperature tempering treatment can effectively eliminate the residual stress in the roller chain, usually 80%-90% of the residual stress can be eliminated. The temperature and time of high-temperature tempering should be precisely controlled according to factors such as the material, size and performance requirements of the roller chain to ensure the heat treatment effect and quality. However, overall high-temperature tempering treatment requires larger heat treatment equipment and the treatment cost is relatively high, but for some roller chain products with strict requirements on residual stress, it is an ideal method to eliminate residual stress.
Local high-temperature tempering: When the roller chain is large in size or complex in shape, and overall high-temperature tempering is difficult, local high-temperature tempering can be used. Local high-temperature tempering is to heat only the weld of the roller chain and the local area near it to eliminate the residual stress in the area. Compared with overall high-temperature tempering, local high-temperature tempering has relatively lower equipment requirements and processing costs, but its effect of eliminating residual stress is not as thorough as overall high-temperature tempering. When performing local high-temperature tempering, attention should be paid to the uniformity of the heating area and the control of the heating temperature to avoid new stress concentration or other quality problems caused by local overheating or uneven temperature.
(IV) Mechanical stretching method
The mechanical stretching method is to apply a tensile force to the roller chain after welding to cause plastic deformation, thereby offsetting the compressive residual deformation generated during the welding process and achieving the purpose of reducing residual stress. In actual operation, special stretching equipment can be used to set appropriate tensile force and stretching speed according to the specifications and performance requirements of the roller chain to uniformly stretch the roller chain. This method has a good effect on some roller chain products that require precise size control and residual stress elimination, but it needs to be equipped with corresponding stretching equipment and professional operators, and has certain requirements for production sites and process conditions.
(V) Temperature difference stretching method
The basic principle of the temperature difference stretching method is to use the temperature difference generated by local heating to cause tensile deformation in the weld area, thereby reducing residual stress. The specific operation is to use an oxyacetylene torch to heat each side of the roller chain weld, and at the same time use a water pipe with a row of holes to spray water for cooling at a certain distance behind the torch. In this way, a high temperature area is formed on both sides of the weld, while the temperature of the welding area is low. The metal on both sides expands due to heat and stretches the weld area with a lower temperature, thereby achieving the purpose of eliminating some welding residual stress. The equipment of the temperature difference stretching method is relatively simple and easy to operate. It can be flexibly applied at the construction site or production site, but its effect of eliminating residual stress is greatly affected by parameters such as heating temperature, cooling speed, and water spraying distance. It needs to be accurately controlled and adjusted according to actual conditions.
(VI) Vibration aging treatment
Vibration aging treatment uses the effect of vibration mechanical energy to make the roller chain resonate, so that the residual stress inside the workpiece is homogenized and reduced. The roller chain is placed on a special vibration aging equipment, and the frequency and amplitude of the exciter are adjusted to make the roller chain resonate within a certain period of time. During the resonance process, the metal grains inside the roller chain will slip and rearrange, the microstructure will be improved, and the residual stress will gradually decrease. Vibration aging treatment has the advantages of simple equipment, short processing time, low cost, high efficiency, etc., and will not affect the surface quality of the roller chain. Therefore, it has been widely used in roller chain production. Generally speaking, vibration aging treatment can eliminate about 30% – 50% of the residual stress of roller chain welding. For some roller chain products that do not require particularly high residual stress, vibration aging treatment is an economical and effective method for eliminating residual stress.
(VII) Hammering method
The hammering method is a simple and commonly used method to reduce welding residual stress. After the roller chain is welded, when the weld temperature is at 100 – 150℃ or above 400℃, use a small hammer to evenly tap the weld and its adjacent areas to cause local plastic deformation of the metal, thereby reducing residual stress. It should be noted that during the hammering process, it should be avoided in the temperature range of 200 – 300℃, because the metal is in a brittle stage at this time, and hammering can easily cause the weld to crack. In addition, the force and frequency of the hammering should be moderate, and should be adjusted according to factors such as the thickness of the roller chain and the size of the weld to ensure the hammering effect and quality. The hammering method is usually suitable for some small, simple roller chain weldments. For large or complex roller chain weldments, the effect of the hammering method may be limited and needs to be used in combination with other methods.

3. How to choose a suitable residual stress reduction method
In actual production, according to the different situations and requirements of the roller chain, it is necessary to comprehensively consider the advantages and disadvantages, scope of application, cost and other factors of various residual stress reduction methods to select a suitable treatment method. For example, for some high-precision, high-strength, thick-walled roller chains, overall high-temperature tempering may be the best choice; while for some large batches and simple shapes of roller chains, vibration aging treatment or hammering method can effectively reduce production costs and improve production efficiency. At the same time, when choosing a method to reduce residual stress, it is also necessary to fully consider the use environment and working conditions of the roller chain to ensure that the method adopted can meet the performance requirements and quality standards of the roller chain in actual use.
4. The role of reducing residual stress in improving the quality and performance of roller chains
Reducing welding residual stress can significantly improve the fatigue strength of roller chains. When the residual tensile stress in the roller chain is reduced or eliminated, the actual stress level it bears during operation is reduced accordingly, thereby reducing the risk of fracture failure caused by the initiation and expansion of fatigue cracks and extending the service life of the roller chain.
It helps to improve the dimensional stability and shape accuracy of the roller chain. Excessive residual stress can cause the roller chain to deform during use, affecting its matching accuracy with sprockets and other components, and thus affecting the normal operation of mechanical equipment. By reducing residual stress, the roller chain can maintain good dimensional stability and shape accuracy during use, and improve the reliability and accuracy of transmission.
It can reduce the tendency of stress corrosion cracking of roller chains in corrosive environments. Residual tensile stress will increase the sensitivity of roller chains to stress corrosion cracking in corrosive media, and reducing residual stress can effectively reduce this risk, improve the corrosion resistance of roller chains in harsh environments, and broaden their application range.