Hardening without heat treatment

As a production manager, you know the problem: Heat treatment consumes a large portion of your production energy—and ties up valuable capacity. Yet, you often only need localized surface hardness, not through-hardening. In this article, we’ll show you alternative processes that let you save up to 30% of the energy used—without compromising component quality. One process stands out in particular, one that most people do not associate with increased hardness: deep rolling.

The hardness of a material is defined as resistance that opposes the penetration of another body. It is therefore a very fundamental material property when it comes to designing a component to withstand specific loads. Above all, the relationship between hardness and strength is also relevant here. 

Hardness can be required very locally on the surface, or it can be used to increase the strength of the entire component. For this reason, it is important for every designer and production planner to know why a component needs to be hardened in the first place. 

During hardening, a targeted microstructural transformation is induced, which increases the material’s resistance. In steel materials, this process primarily exploits the fact that iron exhibits two different lattice types depending on temperature: body-centered cubic and face-centered cubic. 

Application of heat causes the lattice to expand, creating larger gaps between the iron atoms. Carbon diffuses into these gaps. If the material is then rapidly quenched, the iron lattice contracts back, trapping the carbon. This induces residual stresses in the microstructure and hinders the movement of dislocations. The material becomes harder. 

In conventional heat treatment, a component is heated to a predetermined temperature and held at that temperature. This is followed by controlled quenching and subsequent tempering to relieve the stress in the microstructure and restore some elasticity to the component. 

Advantages of heat treatment

Due to the clear physical relationship between crystal structure and temperature, this process is highly controllable. Simply put, as long as parameters of temperature and holding time are maintained, the desired result is achieved. 

There are various methods for the heating process itself. For example, it can take place in a continuous furnace, or each part can be heated individually using induction. This makes the process highly scalable and suitable for mass production. 

Disadvantages of heat treatment

However, the major drawback of heat treatment is clearly the very high energy consumption. Components must be heated to temperatures exceeding 700°C and held at this temperature for a certain period of time. The amouns of energy required for this are substantial. This is also evident in an LCA (Life Cycle Assessment). Here, the categories “raw material production” and “heat treatment” are usually the main drivers of the CO₂ footprint. In addition, this results in very high energy costs. The Lower Saxony Climate Protection and Energy Agency estimates the energy consumption for heat treatment in a process chain consisting of heating, forming, heat treatment, and machining at 26% [1]. 

Another disadvantage of heat treatment is the lack of flexibility. Especially in the case of continuous furnaces in mass production, since these cannot simply be turned on or off. Continuous utilization must therefore be ensured.

Neben der klassischen Härtung durch eine Wärmebehandlung gibt es eine Reihe von alternativen Verfahren, um die Härte einer Oberfläche zu steigern. Je nach erforderlicher Härte und Einhärtetiefe können unterschiedliche Prozesse genutzt werden. 

However, an increase in hardness can also be achieved by plastically deforming the microstructure without applying heat. Cold working increases dislocation density in the microstructure, which also counteracts dislocation movement and thereby increases hardness and strength. 

A very cost-effective and simple method for this is deep rolling, also known just as rolling. In this process, a roller is pressed against the surface with a defined force, causing localized plastic deformation of the near surface areas. 

This plastic deformation not only induces high compressive residual stresses but also increases the hardness of the component. For example, deep rolling with a hydrostatic deep rolling tool of the HG6 type on C60 steel (AISI 1060) resulted in an increase in surface hardness from 290 HV0.5 to 340 HV0.5. This represents a 17% increase in hardness (Figure 3, left). 

The same studies also demonstrated that this increase in hardness is possible even with pre-hardened steel. For the heat-treated condition (QT), it was shown that hardness can be increased from approximately 755 HV0.5 to 810 HV0.5 (Figure 3, right). It is therefore possible to further improve an already hardened surface [2].

If you would like to learn more about deep rolling, please take a look at one of the two additional articles: 

• Process-reliable deep rolling with hydrostatic rolling tools

• Protect components from wear with hard surfaces

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