Residual stresses in tool steel are a common yet critical issue that can significantly impact the performance and longevity of tools. As a tool steel supplier, I’ve witnessed firsthand the challenges that residual stresses pose to our customers. In this blog, I’ll share some effective methods to relieve residual stresses in tool steel, drawing on both industry knowledge and our own experiences. Tool Steel
Understanding Residual Stresses in Tool Steel
Before delving into the relief methods, it’s essential to understand what residual stresses are and how they form in tool steel. Residual stresses are internal stresses that remain in a material after the removal of external forces. In tool steel, these stresses can be introduced during various manufacturing processes, such as casting, forging, heat treatment, machining, and welding.
Casting involves the solidification of molten metal, which can lead to uneven cooling rates and the formation of residual stresses. Forging, the process of shaping metal through compressive forces, can also introduce stresses due to the deformation of the material. Heat treatment, a crucial step in enhancing the mechanical properties of tool steel, often involves rapid heating and cooling, which can cause thermal stresses. Machining operations, like cutting and grinding, can generate surface stresses, while welding can introduce high residual stresses in the heat – affected zone.
These residual stresses can have detrimental effects on tool steel. They can cause dimensional instability, leading to parts that do not meet the required specifications. In addition, residual stresses can reduce the fatigue strength of the tool, making it more susceptible to cracking and failure under cyclic loading. They can also lower the corrosion resistance of the tool steel, increasing the risk of premature degradation.
Methods to Relieve Residual Stresses
Thermal Stress Relieving
Thermal stress relieving is one of the most widely used methods for reducing residual stresses in tool steel. This process involves heating the tool steel to a specific temperature below its critical transformation temperature and holding it at that temperature for a certain period, followed by slow cooling.
The heating temperature and holding time depend on the type of tool steel, the magnitude of the residual stresses, and the size and shape of the part. Generally, for most tool steels, the stress – relieving temperature ranges from 550°C to 700°C (1022°F to 1292°F). During this time, the atoms in the steel have enough energy to rearrange themselves, reducing the internal stresses.
Slow cooling is crucial to prevent the formation of new residual stresses. A cooling rate of about 20°C to 50°C per hour (36°F to 90°F per hour) is typically recommended. This gradual cooling allows the material to contract uniformly, minimizing the development of thermal stresses.
One advantage of thermal stress relieving is its effectiveness in reducing both macroscopic and microscopic residual stresses. It can also improve the dimensional stability of the tool steel, making it more suitable for precision applications. However, it should be noted that thermal stress relieving can slightly reduce the hardness of the tool steel, so it’s necessary to balance the stress reduction with the required mechanical properties.
Vibratory Stress Relieving
Vibratory stress relieving is a non – thermal method that uses mechanical vibrations to reduce residual stresses in tool steel. In this process, a vibrating device is attached to the tool steel part, and the part is subjected to a specific frequency and amplitude of vibration for a certain period.
The vibrations cause the dislocations in the crystal lattice of the tool steel to move and rearrange, which helps to relieve the residual stresses. The frequency and amplitude of the vibration are carefully selected based on the material properties, size, and shape of the part.
Vibratory stress relieving has several advantages. It is a relatively quick and cost – effective method, as it does not require the high energy consumption associated with thermal stress relieving. It can also be used on large or complex – shaped parts that may be difficult to heat uniformly in a furnace. Additionally, it does not cause any significant changes in the mechanical properties of the tool steel, such as hardness or strength.
However, the effectiveness of vibratory stress relieving can be affected by factors such as the initial stress distribution in the part and the accuracy of the vibration parameters. It may not be as effective as thermal stress relieving in reducing high – level residual stresses, especially in parts with complex stress states.
Mechanical Stress Relieving
Mechanical stress relieving involves applying an external force to the tool steel part to induce controlled plastic deformation, which can redistribute the residual stresses. This can be achieved through methods such as shot peening, roller burnishing, or stretching.
Shot peening is a process in which small spherical particles (shots) are blasted onto the surface of the tool steel at high velocity. The impact of the shots creates a compressive stress layer on the surface, which can counteract the existing tensile residual stresses. Shot peening can also improve the fatigue resistance of the tool steel by inhibiting crack initiation and propagation.
Roller burnishing is another mechanical stress – relieving method. In this process, a hardened roller is pressed against the surface of the tool steel while the part is rotated. The rolling action of the roller causes plastic deformation of the surface layer, resulting in the redistribution of residual stresses. Roller burnishing can improve the surface finish of the tool steel as well as its stress state.
Stretching is a method that involves applying a tensile force to the tool steel part. By stretching the part within its elastic – plastic range, the residual stresses can be reduced. However, this method requires careful control of the stretching force to avoid over – deformation and damage to the part.
Importance of Stress Relief for Our Customers
As a tool steel supplier, we understand the importance of providing our customers with high – quality tool steel that is free from excessive residual stresses. Our customers rely on our tool steel to manufacture a wide range of tools, from cutting tools and dies to molds and punches. By ensuring that our tool steel has low residual stresses, we can help our customers improve the performance and reliability of their tools.
For example, in the manufacturing of cutting tools, residual stresses can cause the tool to wear unevenly, leading to poor cutting performance and reduced tool life. By using stress – relieved tool steel, our customers can expect more consistent cutting results and longer tool life, which can ultimately reduce their production costs.
In the case of dies and molds, residual stresses can cause dimensional changes during the manufacturing process, resulting in parts that do not meet the required tolerances. Stress – relieving the tool steel can help maintain the dimensional stability of the dies and molds, ensuring the production of high – quality parts.
Our Role as a Tool Steel Supplier
At our company, we take several steps to ensure that the tool steel we supply has low residual stresses. First, we carefully select our raw materials from reliable sources to minimize the initial stress levels in the steel. During the manufacturing process, we use advanced techniques to control the heat treatment and forming operations, which can significantly reduce the generation of residual stresses.
In addition, we offer stress – relieving services to our customers. Our experienced technicians use state – of – the – art equipment to perform thermal, vibratory, or mechanical stress – relieving processes, depending on the specific requirements of the tool steel and the application. We also provide detailed quality control reports to our customers, indicating the reduction of residual stresses and the improvement of the material properties.
Conclusion and Call to Action
Relieving residual stresses in tool steel is a crucial step in ensuring the performance and longevity of tools. By using methods such as thermal stress relieving, vibratory stress relieving, and mechanical stress relieving, we can effectively reduce the internal stresses in tool steel and improve its dimensional stability, fatigue strength, and corrosion resistance.
Alloy Steel As a tool steel supplier, we are committed to providing our customers with high – quality tool steel and comprehensive stress – relieving solutions. If you are in need of tool steel for your manufacturing processes or want to learn more about our stress – relieving services, we encourage you to contact us for further discussion. We look forward to working with you to meet your tool steel needs and help you achieve better results in your production.
References
- ASM Handbook Volume 4: Heat Treating. ASM International.
- Manufacturing Processes for Engineering Materials by S. Kalpakjian and S. R. Schmid. Pearson Education.
- Metals Handbook Desk Edition, 3rd Edition. ASM International.
Jiangsu Cunrui Metal Products Co., Ltd.
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