Table of Contents
Introduction
If you’ve ever dealt with hardened steel cracking after tempering, you know how frustrating and costly it can be. Tempering cracks occur when hardened steel undergoes rapid heating or cooling, leading to excessive stress that the material cannot withstand. These cracks significantly weaken steel, making it unreliable for structural applications, machinery, or tools.
This issue is particularly common in high-alloy steels and high-speed steels, where the risk of cracking increases due to their unique composition. However, proper heat treatment techniques can reduce or even eliminate this problem.
In this article, we’ll cover what causes tempering cracks, which materials are most at risk, how to prevent them, and the best methods for detection.
What Are Tempering Cracks?
Tempering cracks are fractures that appear in hardened steel during the tempering process due to internal stresses caused by rapid temperature changes or microstructural transformations. These cracks can occur in different forms and typically appear at sharp edges, corners, or highly stressed regions of a component.
There are two main types of tempering cracks:
- Cracks caused by rapid heating: Occur when the outer surface expands too quickly while the core remains cooler, creating internal stress.
- Cracks caused by rapid cooling: Form when structural changes happen too fast, making the material brittle and prone to fracturing.
In general, tempering cracks do not occur under normal tempering conditions. However, when high-alloy steels undergo rapid temperature changes, the likelihood of cracking increases significantly.
What Causes Tempering Cracks?
Heating Too Fast
When steel is heated too quickly for tempering, especially with flame heating or induction heating, the outer layers absorb heat much faster than the core. This creates an imbalance, where the outer surface tries to expand while the inside remains cool. The difference in thermal expansion generates high tensile stress, which can lead to cracking.
Cooling Too Fast
If steel cools too rapidly after tempering, it undergoes sudden structural changes that increase internal stress levels. This is particularly problematic in high-alloy steels, which have complex microstructures that react aggressively to rapid cooling. The faster the phase transformation occurs, the greater the stress buildup, ultimately leading to brittle fractures similar to quenching cracks.
Decarburization
Decarburization occurs when carbon is lost from the surface of the steel, usually during earlier heat treatment processes. A decarburized layer has reduced hardness and does not transform properly during tempering. Meanwhile, the inner core expands and contracts as expected, but the soft outer layer cannot handle the stress difference. This results in surface cracks that weaken the steel.
Which Steels Are More Prone to Tempering Cracks?
The risk of tempering cracks depends on the composition and hardenability of the steel. Some steels are naturally more resistant, while others require careful heat treatment to avoid cracking.
- Carbon steels: Generally low risk, as they have lower hardenability and less tendency for extreme stress buildup.
- High-speed steels: High risk, due to their high alloy content, which makes them more sensitive to temperature changes.
- High-alloy steels: High risk, especially when cooled quickly after tempering. The presence of elements like chromium, vanadium, and molybdenum increases their susceptibility to cracking.
High-alloy steels undergo more aggressive structural transformations compared to plain carbon steels. This means they require a slower, more controlled tempering process to prevent stress buildup and crack formation.
How to Prevent Tempering Cracks?
Heat the Steel Gradually
One of the most effective ways to prevent tempering cracks is to avoid rapid temperature changes. If the steel is heated too quickly, the surface expands much faster than the core, creating internal stress.
- Use preheating steps to gradually raise the temperature.
- Avoid direct flame heating for thick components, as it leads to uneven heating.
- For large parts, heat in stages to minimize thermal gradients.
Control the Cooling Process
Cooling should be gradual and controlled to prevent sudden stress buildup. Different steels require different cooling methods:
- Carbon steels: Air cooling is usually sufficient.
- High-speed and high-alloy steels: Furnace cooling or slow air cooling is preferred.
- For critical parts: Holding the steel at a temperature slightly above the martensite start (Ms) point before cooling can reduce stress.
Remove the Decarburized Layer
If steel has a decarburized surface, it becomes weak and prone to cracking. Before tempering:
- Check for decarburization using metallographic analysis.
- Remove the soft layer by machining, grinding, or acid pickling.
- Ensure uniform hardness before tempering to avoid uneven stress distribution.
Optimize Heat Treatment Parameters
Each steel grade requires specific heating and cooling rates. Overheating or under-tempering can both increase the risk of cracking.
- Follow recommended temperature ranges for each steel type.
- Control holding times to ensure complete transformation.
- Use proper quenching techniques before tempering, as improper quenching can make the steel more prone to cracking later.
How Do Tempering Cracks Affect Steel Performance?
Tempering cracks significantly reduce steel quality and reliability. If left undetected, they can lead to mechanical failure, reduced lifespan, and safety risks. The key impacts include:
- Loss of Strength and Toughness. The steel loses its load-bearing capacity.
- Stress Concentration Points. Cracks act as weak spots, increasing the chance of fracture.
- Reduced Fatigue and Corrosion Resistance. Cracks expose internal surfaces to environmental damage.
If tempering cracks appear in critical machine components, tools, or structural parts, they can cause catastrophic failure.
How to Detect Tempering Cracks?
Visual Inspection
The simplest method is to look for visible cracks using the naked eye or a magnifying glass. However, this method only works for surface cracks.
Magnetic Particle Testing (MT)
This non-destructive testing (NDT) method uses magnetic fields to reveal cracks near the surface. It’s effective for detecting fine cracks in ferromagnetic steels.
Ultrasonic Testing (UT)
If you suspect internal cracks, ultrasonic testing is a highly accurate way to find defects. It uses sound waves to detect fractures deep within the steel.
Metallographic Analysis
For a detailed investigation, a sample can be cut, polished, etched, and examined under a microscope to study the microstructure and crack formation.
Conclusion
Tempering cracks weaken steel and shorten its lifespan. By controlling heating and cooling rates, removing decarburized layers, and optimizing heat treatment, you can prevent cracks and improve steel performance. If you’re facing issues with tempering cracks or need expert guidance on optimizing your heat treatment process, Alloyxpert is here to help.
FAQs
Can tempering cracks be repaired?
No, tempering cracks cannot be repaired. Once a crack forms, the steel’s structural integrity is permanently compromised. The best approach is prevention. If a part has tempering cracks, it should be discarded or remanufactured.
Why do high-speed steels crack more easily?
High-speed steels contain large amounts of alloying elements, such as tungsten, molybdenum, and vanadium, which make their microstructure more sensitive to temperature changes. These steels require slow, controlled tempering to prevent cracking.
How do I know if my steel is at risk?
Check the steel’s composition and hardenability. The more alloying elements a steel contains, the greater the risk. If the steel is high in chromium, vanadium, or molybdenum, it requires careful heat treatment.
Read More:
How to Identify Metal Cracks in Raw Material, Heat Treatment, and Forging?