Hardenability and hardenability are fundamental characteristics that directly affect the performance properties of materials after heat treatment. Without understanding these properties, it is impossible to create reliable and durable steel products.
What is hardenability and hardenability?
Hardenability — the ability of the material to achieve high hardness and strength after hardening. This parameter is largely determined by the chemical composition of the alloy. For example, carbon increases hardness, but excess carbon can reduce plasticity.
Hardenability characterizes the depth of penetration of hardening into steel. It depends on factors such as chemical composition, austenite grain size, and cooling rate. High hardenability ensures uniform hardness throughout the product, which is especially important for large parts and structures of complex shape.
These properties are key in steel production for industries such as oil and gas industry, aviation, construction and mechanical engineering, where high requirements for the strength and durability of materials do not allow compromises.
Why is this important?
Hardenability and hardenability play a crucial role in ensuring product performance:
- Hardenability makes it possible to obtain products with the necessary wear resistance, plasticity and strength. This is especially important for tools and parts operating under high loads.
- Hardenability ensures that even massive structural elements will have uniform hardness. This reduces the risk of weak zones that can damage the product.
These characteristics determine success heat treatment and the quality of the finished product.
Steel hardening process
Hardening is a complex technological process that includes several stages:
- Heating. The heating temperature depends on the composition of the alloy. For pre-eutectoid steels (carbon up to 0.8%), the heating temperature should exceed the critical point Ac3 by 30—50°C. For eutectoid steels (carbon above 0.8%), the temperature is selected above the Acm point by the same 30—50°C.
- Excerpt. The heating time depends on the size of the part and the type of furnace. Usually, the exposure time is 1-2 minutes for every millimeter of product thickness.
- Cooling. Rapid cooling prevents austenite from breaking down into ferrite and cementite, which increases the material's hardness. Three cooling methods are used:
- In one cooler. Immerse the part in water or oil. This method is suitable for carbon steels.
- Isothermal cooling. Provides an even distribution of hardness.
- Step-by-step cooling. It reduces internal stresses, improving durability.
- Vacation. After hardening, the material is tempered to relieve internal stresses and increase plasticity. Depending on the temperature, the vacation is divided into:
- low (150—200°C);
- medium (350—450°C);
- high (500—650°C).
Careful adherence to hardening technology makes it possible to achieve optimal steel characteristics.
Factors affecting hardenability and hardenability
Chemical composition
- Carbon. It increases hardness and strength, but when the content is high, it leads to the formation of carbides, which reduces the plasticity of the material.
- Alloying elements. Chrome, nickel, manganese and molybdenum help improve the structure, increase the hardness and hardenability of alloys.
Cooling rate
Rapid cooling contributes to the formation of a solid martensite structure. Slow — leads to a decrease in hardness due to the breakdown of austenite into ferrite and cementite.
The cooling rate is selected in accordance with the chemical composition and required mechanical properties. Carbon steels usually use rapid cooling in water or oil, while alloyed steels use slower cooling.
Steel structure before hardening
The fine-grained structure ensures uniform hardening and high strength. Coarse-grained alloys may have reduced hardenability due to the uneven distribution of carbon and alloying additives. To improve the structure, preliminary heat treatment is carried out.
Defects
Pores, cracks and non-metallic inclusions reduce the quality of hardening and impair the quality of hardening. Therefore, before processing steels and alloys carefully check for defects.
Methods for assessing hardenability
Hardenability reflects steel's ability to achieve high hardness and strength after hardening. The following methods are used to evaluate it:
Hardness measurement
This is one of the most accessible and simple methods for assessing hardenability. The process includes:
- Sample hardening.
- Hardness measurement after hardening using HRC (Rockwell hardness) or HV (Vickers hardness) scales.
The higher the hardness after hardening, the better the hardenability of steel.
Tensile test
This method involves mechanical tests on tensileto define the following parameters:
- Tensile strength — the maximum stress that leads to the destruction of the material.
- Yield strength — stress, at which the deformation continues to increase without increasing the load.
High values of these characteristics indicate good hardenability.
Differential scanning calorimetry (DSC)
DSC is based on measuring the heat fluxes released or absorbed by the material during phase transitions:
- It allows you to determine the temperature of the beginning and end of the martensitic transformation.
- It helps to assess the efficiency of hardening and identify optimal heat treatment conditions.
Microstructure analysis
The method includes studying the structure steels after hardening using a microscope. The focus is on:
- The size of the austenite grain before hardening, which affects the uniformity of hardenability.
- The presence and distribution of phases (martensite, ferrite, carbides) in hardened steel.
The method is particularly effective for analyzing alloys with the addition of alloying elements that affect the phase composition.
The effect of chemical composition on hardenability
Chrome (Cr)
- Increases hardenability by increasing the stability of the austenitic structure.
- It increases hardness and strength.
- It improves corrosion resistance, which makes chromium-containing steels popular in aggressive environments.
Nickel (Ni)
- Improves plasticity and viscosity, which reduces the risk of brittle destruction.
- It contributes to the uniform distribution of carbides.
- It increases impact strength and fatigue strength, which is important for critical structures.
Manganese (Mn)
- Reduces oxidation steel during melting and heat treatment.
- Forms solid solutions with carbon, increasing hardenability.
- Improves wear resistance and weldability.
Molybdenum (Mo)
- Forms stable carbides, which increase the hardness of the alloy.
- It increases heat resistance and strength at high temperatures.
- It reduces the risk of hardening cracks due to the uniform distribution of stresses.
Carbon (C)
- It is the main element that increases the hardness and strength of steel.
- A high carbon content improves hardenability, but excess can reduce plasticity.
The optimal combination of elements
The balance between the content of carbon and alloying elements makes it possible to achieve the required hardenability depth and ensure the high performance properties of steel.
Methods for determining hardenability
Hardenability is especially important for products of large size or complex shapes.
End hardening method (GOST 5657-69)
The main steps include:
- Sample preparation. A cylindrical sample is made from the test material, usually of a standard size.
- Heating. The sample is heated to a temperature typical for hardening of a particular grade (usually above the critical point to completely transfer the material to the austenitic state).
- Cooling. A stream of water or oil is directed to the end of the heated sample. This creates a cooling gradient, where the end cools as quickly as possible, and as you move away from it, the cooling rate decreases.
- Hardness measurement. After quenching, the hardness of the material is measured at various points along the sample axis.
- Building a graph. Based on the measurements, a curve is drawn between hardness and the distance to the end of the sample. This makes it possible to determine the depth of hardenability of steel.
Using test bars
To assess hardenability, special samples are used — test bars. This method involves the following steps:
- Making bars. Workpieces of standard shape and size are made of test steel.
- Hardening. Samples are subjected to a standard hardening process, which includes heating and cooling.
- Structure and hardness analysis. After hardening, the change in hardness over the sample section is studied. The deeper the high hardness is maintained, the better the hardenability.
Products and capabilities of the PZPS plant
Properly selected heat treatment modes make it possible to produce products with the required mechanical properties, which is especially important for carbon steels 60S2A, 65G, 70, 70S2HA, U8A, U10A.
Also strict control at all stages of heat treatment, it is necessary during production:
Thanks to modern equipment and high quality employee qualifications PZPS guarantees that products comply with modern quality standards. With us you can buy cold-rolled soft alloy strip 27KH, as well as other special steels and precision alloys necessary for the successful implementation of the most complex projects.
