Hardness of steels and alloys: definition, measurement methods and influencing factors

Hardness is one of the main mechanical properties that directly affects the machinability of steels and alloys. Materials with high hardness require the use of more powerful processing equipment and tools, which can significantly increase production time and the final cost of finished products. Excessive softness can lead to surface defects, such as burrs and scratches, during processing.

Determination of hardness and its importance in industry

Hardness is a mechanical property that determines the ability of a material to resist plastic deformation, indentation, abrasion and destruction under local impact. When choosing steels and alloys for critical structures and important equipment components, this parameter plays a crucial role. In industries such as mechanical engineering, metallurgy, aviation and automotive industries, the wear resistance, strength and durability of finished products directly depend on the level of hardness.

Tool steels used for the production of drills, cutters and lathes must have a high hardness in order to maintain cutting capacity under heavy loads. IN automobiles and aircraft, engine and transmission elements must combine optimal levels of hardness and viscosity, which ensures that components and parts are resistant to wear and mechanical loads. However, excessive hardness can reduce the workability of the material and lead to increased brittleness.

Hardness measurement methods

In modern industry, several methods are used to analyze steels and alloys. They are all based on pressing indenters into the surface of the material or scratching the control sample.

Brinell method (HB)

Used for soft and medium-hard materials, including structural and carbon steels.

The determination of Brinell hardness relates to the methods of indentation and involves determining the diameter properties of the print left after removing the applied load.

Measurement principle:

1. The prepared sample is fixed on a solid surface.
2. A hardened steel or carbide ball with a diameter of 2.5, 5 or 10 mm is pressed into the surface.
3. Using a special press, a static load of 625 H to 30,000 H is applied, depending on the material being tested.
4. After holding for a certain time (usually 10—30 seconds), the load is removed, and the diameter of the print is measured with high accuracy using a magnifying glass or microscope.
5. Hardness is calculated as the ratio of the applied load to the area of the spherical print.

The advantages of the method:

• It is suitable for studying heterogenous and coarse-grained materials.
• Ease of execution and ease of use.
• High accuracy and reproducibility of results.

Disadvantages:

• Not suitable for very hard materials.
• It requires careful surface preparation for accurate measurements.

Rockwell method (HR)

It is based on measuring the depth of pressure of an indenter (a diamond cone or a steel ball) into the sample surface when exposed to two conceptual loads. This method is one of the most common because of its simplicity and speed.

Test steps:

1. The prepared sample is placed on a solid surface.
2. To eliminate the effect of surface irregularities, a preload is applied to the sample (P₀).
3. Using the indenter, the main load (P) is applied. For soft materials, a diamond cone is used, for hard materials, a steel ball is used.
4. After removing the load, the residual indentation depth (h) is recorded.

Hardness is defined as the difference between the depth of the print after the indenter is pressed under the influence of the main and preliminary loads. The studied property is expressed in units of Rockwell hardness.

Measurement scales:

• HRA — a diamond cone is used as an indenter, the maximum load is 60 kgf. It is used for particularly hard materials such as carbides, cemented steels, heat-resistant alloys.
• HRB — indenter — steel ball, test load — 100 kgf. The scale is used for medium-hard metals: copper and aluminum alloys, mild steels, ductile iron.
• HRC — involves the use of an indenter in the form of a diamond cone and a full test load of 150 kgf. HRC is considered a universal scale and is widely used to study high-carbon steels and alloys that have passed heat treatment.

The advantages of the method:

• Easy and convenient measures
• Repeatability of results and high accuracy in determining hardness.
• The ability to measure the properties of a wide range of materials.

Disadvantages:

• It requires thorough preparation of the sample surface.
• Under high loads, it can change the structure of samples, so it has limitations when measuring thin materials.
• When using different scales, it can be difficult to interpret the results.

Vickers method (HV)

It belongs to static methods and is used to study the ability of a material to resist plastic deformation. It allows you to determine the hardness of small parts, thin-walled products, welds, high-strength coatings. In the Vickers method, the indenter is a tetrahedral diamond pyramid with a dihedral angle at the top of 136°.

Test principle:

1. The diamond indenter is pressed into the material under a load of up to 100 kgf for 10-15 seconds.
2. After the load is removed, an imprint remains on the sample surface.
3. Hardness is defined as the ratio of the load to the area of the pyramidal print.

Hardness is expressed in HV units and is determined at a load of 30 kgf and a holding time of 10 to 15 seconds (in accordance with GOST 2999-75). If the test conditions differed from the standard ones, the letters indicate the applied load and the holding time. For example, 500 HV 60/20 means that the material was tested at a load of 60 kgf and a holding time of 20 seconds.

The advantages of the method:

• High accuracy and reproducibility of results.
• The ability to study fairly thin samples.
• The ability to apply the method to analyze various steels and alloys, ceramics, polymers and other materials.

Cons:

• The duration of the test.
• The need for thorough sample surface preparation.

Factors affecting hardness

The following factors influence the hardness of steels and alloys:

• Chemical composition: alloying elements significantly change the physical and mechanical properties of steel. For example, carbon contributes to the formation of cementite, which hardens steel, but excess carbon impairs plasticity. Chromium increases hardness and corrosion resistance, molybdenum increases strength at high temperatures, and vanadium reduces grain and improves tensile strength. Nickel and copper improve the plasticity of alloys.
• Heat treatment — when hardened, austenite turns into martensite, which increases the hardness of the material, but makes it brittle. To reduce fragility, tempering is used. Annealing improves plasticity and relieves internal stresses. During annealing, the metal is slowly cooled, which contributes to the formation of a coarser structure and a decrease in hardness.
• Mechanical processing, such as forging, rolling, stamping, as well as subsequent metal processing, can significantly increase the hardness of the material. These processes change the structure of steels and alloys and increase their mechanical characteristics due to dislocation hardening.
• Grain size in metal, it directly affects its hardness and other mechanical properties. This phenomenon is due to an effect known as the “grain boundary effect”. The smaller grain size helps to increase the hardness of the material, since smaller grains make it difficult for the dislocations that are responsible for plastic deformation to move.
• Unwanted imperficities, such as sulfur, phosphorus, oxygen and nitrogen, can advertently affect the hardness and other mechanical properties of metals. These elements, as a rule, form harmful inclusions that weaken the structure of the material and increase its tendency to brittleness.
• Surface condition — the surface hardness of the material may differ significantly from the hardness of its bulk, depending on the processing methods. After grinding, polishing, or heat treating the surface, compressive stresses may occur that increase the hardness of the upper layer.

To create high-quality materials that meet modern industry requirements, it is necessary to carefully control all these parameters in order to achieve an optimal combination of hardness, plasticity and wear resistance.

PZPS products

The St. Petersburg Precision Alloy Plant produces wide range high quality materials, including:

• precision alloys: 40KHNM, 36NHTYU, 17HNGT;
• corrosion-resistant steels: 12X18H10T, 12X18N9, 10X17N13M3T;
• precision alloys with a given temperature coefficient of linear expansion: 29NK, 36N, 42N;
• spring and tool steels: U8A, U10A, 65G, 60S2A, 70S2HA;
• heat-resistant alloys: HN78T, 20X13.

All materials comply with international quality standards and are certified. For more information or collaboration, please contact us at specified phones or leave a request on the site. Our experts will help you choose materials that are ideally suited to your project requirements.

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