Effect of Heating on Stainless Steel: Deterioration Without Hardening

Stainless steel is widely known for its remarkable resistance to corrosion and wear, making it an ideal material for a multitude of applications. While carbon steel can be hardened through a variety of thermal treatments, stainless steel does not undergo the same kind of hardening. However, does this mean that heating stainless steel does not cause any detrimental effects? This article aims to explore the effects of heating on stainless steel, particularly highlighting the common issues such as oxidation, scaling, phase changes, and thermal expansion.

Effects of Heating on Stainless Steel

Heating stainless steel, even though it cannot be hardened in the same manner as carbon steel, still can be subject to various negative impacts:

Oxidation

One of the primary concerns when heating stainless steel is oxidation. At high temperatures, stainless steel undergoes a reaction with oxygen in the air, leading to the formation of a chromium oxide layer. However, if this protective layer is compromised, the stainless steel is more prone to corrosion. This phenomenon is more noticeable in grades with lower chromium content, as the protective layer is not as robust.

Scaling

At elevated temperatures, stainless steel can develop a scale on its surface, leading to a change in its appearance and potentially altering its properties. This scale can be particularly problematic in welded joints or areas exposed to high temperatures, as it can create a thin, brittle layer that can crack or flake off over time.

Phase Changes

Certain grades of stainless steel can undergo phase changes when subjected to high temperatures. For example, some austenitic stainless steel grades can become less ductile and more brittle when rapidly cooled after being heated. This phase change can severely impact the material's mechanical properties and is a significant concern in applications that require high plasticity and toughness.

Thermal Expansion

Heating causes stainless steel to undergo thermal expansion, which can result in stress within structures or components if they are constrained. This stress can lead to potential damage, such as cracking or warping, especially in areas where the material is not allowed to expand freely.

Softening

Exposure to prolonged high temperatures can cause softening of the stainless steel, reducing its mechanical strength and potentially compromising its structural integrity. Softening is particularly concerning in applications that rely on rigid and consistent mechanical properties.

In summary, while stainless steel does not harden in the same way as carbon steel, it is still subject to several negative effects when exposed to high temperatures. These effects can lead to a considerable reduction in the material's performance and durability, making it essential to carefully consider the application and operating conditions when using stainless steel in heated environments.

Surface Hardening of Stainless Steel

Focusing on specific grades of stainless steel, such as ferrotic and austenitic steels, these materials are not susceptible to warmth treating in the traditional sense. However, certain techniques can still achieve localized surface hardening without the need for traditional heat treatment processes.

Low Temperature Carburizing Techniques

For austenitic grades of stainless steel, a process known as kolsterising can be utilized. This involves low-temperature carburization, which provides a surface case depth of 10-50μm and hardness ranging from 100-1200HV0.05. The process does not introduce a phase change, making it ideal for maintaining the material's integrity and corrosion resistance.

This technique not only improves wear resistance but also eliminates galling and other surface-related issues. Additionally, the process does not alter the color, surface roughness, dimensions, or magnetism of the material, ensuring that the original aesthetics and properties remain intact. More importantly, the corrosion resistance of the material is largely unaffected due to the low temperature used in the process.

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