Hardness of metals and alloys. What does it affect? How to increase the hardness of a material?

How is hardness measured and how is it designated?

For most hardness measurement methods, the basic unit of measurement is kgf/mm2

However, you should understand that there are methods with their own unit.

The designation of hardness also depends on the method.

The letter “H” always denotes “hardness” (from the English Hardness), and then the letters indicating the determination method are indicated. The most popular designations:

• HB – Brinell method (pressing a steel ball)
• HRA - Rockwell method, A scale (diamond or steel cone indentation)
• HRB – Rockwell method, B scale
• HRC - Rockwell method, C scale
• HV – Vickers method (diamond pyramid indentation)
• HSD – Shore hardness, etc. (rebound method)

see also

What are the requirements for the product to be measured?

Hardness is directly proportional to the load to determine it. High hardness – high load.

The more accurate the method, the higher the requirements for preparing the surface of the product. The surface of the product on which hardness is determined must meet a number of requirements:

1. The thickness of the sample must be at least 10 times the depth of penetration of the tip after removing the main force.
2. At the inspection site, it must be cleaned to a shine, be smooth and flat, and must be free of scale, rust, oil, fat and paint contamination, potholes and scratches. Roughness Ra is not more than 2.5 µm according to GOST 2789, unless there are other requirements of regulatory and technical documentation.
3. The surface on which the sample “lays” on the object stage of the device must also be clean and level. Both surfaces must be parallel to each other.
4. The product must be securely fastened, excluding the possibility of displacement of the sample relative to the axis of load application.

Recommendations

1. Vredenberg, Fredrik; P.L. Larsson (2009). "Scratches on metals and polymers: experiments and numbers." Wear
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   266
   (1–2): 76. doi:10.1016/j.wear.2008.05.014.
2. Hoffman Scratch Hardness Tester Archived 2014-03-23 ​​on the Wayback Machine. byk.com
3. Allen, Robert (December 10, 2006). "Guide to Rebound Hardness and Scleroscopy Test". Archived from the original on 2012-07-18. Retrieved 2008-09-08.
4. "Novotest".
5. Jandron, Michel (25 August 2005). "Diamonds are not forever." World of Physics
   . In the archive from the original dated 02/15/2009.
6. San Miguel, A.; Blase, P.; Blase, X.; Melinon, P.; Perez, A.; Itie, J.; Polian, A.; Reny, E.; and others. (1999-05-19). "High-pressure behavior of silicon clathrates: a new class of low-compressibility materials." Physical Review
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   83
   (25): 5290. Bibcode:1999ПхРвЛ..83.5290С. Doi:10.1103/PhysRevLett.83.5290.
7. ^ a b
   Haasen, P. (1978). Metallurgy of metals. Cambridge [English]; New York: Cambridge University Press.
8. Samuel, J. (2009). Introduction to Materials Science Handbook
   . Madison, WI: University of Wisconsin-Madison.
9. Smedskjaer, Morten M.; John C. Mauro; Yuanzheng Yue (2010). "Prediction of glass hardness using temperature-dependent constraint theory." Phys.
   Rev. Lett .
   105
   (11): 2010. Bibcode:2010PhRvL.105k5503S. Doi:10.1103/PhysRevLett.105.115503. PMID 20867584.
10. Leslie, W. C. (1981). Metallurgy of steels. Washington: Hempisphere Pub. Corp., New York: McGraw-Hill, ISBN 0070377804.

What methods exist for determining hardness?

Conventionally, all methods can be divided into 3 groups:

1. Indentation (implementation) methods
2. Scratching Methods
3. Elastic rebound methods

Indentation (implementation) methods. The meaning of the methods is to press a so-called indenter - a solid object of a certain shape (usually a steel ball or a diamond pyramid) into the test metal with a certain force. After indentation, the diameter (for a ball) or depth (for a pyramid) of the resulting imprint is measured.

In this case, hardness is defined as the ratio of the load to the area of ​​the indentation after indentation.

The most common are the Brinell (HB) and Rockwell (HRA, HRB, HRC) methods.

Indentation thickness measurement methods:

1. Brinell device
2. Rockwell device
3. Vickers device
4. Ludwick method
5. Hertz method
6. Drozd method
7. Shor's Monotron
8. Berkovich method
9. Egorov's method
10. Khrushchev's method
11. Leeds method
12. Zeiss-Hahnemann microhardness tester
13. PMT-2, PMT3 (Khrushchov, Berkovich)
14. Emerson, Knoop, Peters method

Scratching methods. Simple methods. If the tip used to make the scratch leaves a mark on the metal being tested, then the hardness of the metal is less than the hardness of the tip. In this case, the hardness of the tip is initially known (corundum, diamond, gypsum and other tips are used). The most popular is the Mohs method.

Scratching methods:

1. Mohs test
2. Martens device
3. Bierbaum microcharacterizer
4. File test, Barba
5. Hankins device
6. PMT-3 (Berkovich)
7. PMT-3 (Grigorovich)
8. O'Neill sclerometer

Elastic rebound methods. Rarely used. The firing pin falls freely onto the test surface from a fixed height. Under the action of elastic recoil of the material, the firing pin rebounds to a certain height. The hardness of the material is proportional to the height of the rebound. The most popular is Shor's method.

Elastic rebound methods:

1. Shore Scleroscope
2. Martel method
3. Nikolaev vertical pile driver
4. Shopper spring device
5. Bauman spring device
6. Poldi device
7. Walzel pendulum pile driver
8. Herbert pendulum
9. Kuznetsov pendulum sclerometer

Physics

Diagram of a stress-strain curve, showing the relationship between stress (force applied per unit area) and stress or strain in a ductile metal.
In solid mechanics, solids typically have three responses to force, depending on the force and the type of material:

• They exhibit elasticity - the ability to temporarily change shape, but return to the original shape when pressure is removed. "Hardness" in the elastic range—a small temporary change in shape under a given force—is known as stiffness in the case of a given object, or high modulus of elasticity in the case of a material.
• They exhibit plasticity - the ability to constantly change shape in response to force, but remain integral. The yield point is the point at which elastic deformation gives way to plastic deformation. Deformation in the plastic range is nonlinear and is described by a stress-strain curve. This response gives the observable scratch and indentation hardness properties as described and measured in materials science. Some materials exhibit both elasticity and viscosity during plastic deformation; this is called viscoelasticity.
• They fracture - split into two or more parts.

Strength is a measure of the range of elasticity of a material, or the range of elasticity and ductility combined. This is quantified as compressive strength, shear strength, tensile strength depending on the direction of the forces involved. Incredible force is an engineering measure of the maximum load that a piece of a certain material and a certain geometry can withstand.

Brittleness in technical use is the tendency of a material to fracture with very little or no predetectable plastic deformation. Thus, from a technical point of view, a material can be either brittle or strong. In everyday use, the term "brittleness" usually refers to the tendency to break under the influence of small forces, which manifests itself in both brittleness and lack of strength (in the technical sense). For perfectly brittle materials, the yield strength and ultimate strength are the same because they do not experience noticeable plastic deformation. The opposite of brittleness is ductility.

The durability of a material is the maximum amount of energy it can absorb before breaking, which is different from the amount of force that can be applied. The toughness of brittle materials is usually low because elastic and plastic deformations allow the materials to absorb large amounts of energy.

Hardness increases with decreasing particle size. This is known as the Hall-Petch relationship. However, below the critical grain size, the hardness decreases with decreasing grain size. This is known as the inverse Hall-Petch effect.

The hardness of a material to deformation depends on its microstrength or small scale. shear modulus in any direction, not in any stiffness or stiffness properties such as bulk modulus or junior modulus. Stiffness is often confused with hardness.[5][6] Some materials are harder than diamond (such as osmium), but not harder and are prone to chipping and flaking in a scaly or needle-like form.

Mechanisms and theory

An image of a crystal lattice showing the planes of the atoms.
The key to understanding the mechanism of hardness is understanding the metal's microstructure, or the structure and arrangement of atoms at the atomic level. In fact, the most important metallic properties important for the production of modern products are determined by the microstructure of the material.[7] At the atomic level, the atoms in a metal are organized into an ordered three-dimensional array called a crystal lattice. In reality, however, a given metal sample will probably never contain a consistent single crystal lattice. A given sample of metal will contain many grains, with each grain having a fairly consistent array pattern. On an even smaller scale, each grain contains irregularities.

There are two types of irregularities at the grain level of the microstructure that are responsible for the hardness of the material. These irregularities are point and linear defects. A point defect is an irregularity located at a single lattice point within the overall three-dimensional lattice of the grain. There are three main disadvantages. If there is no atom in the array, a vacancy deficiency is formed. If there is another type of atom at a lattice site that would normally be occupied by a metal atom, a substitutional defect is formed. If an atom exists in a node that normally should not exist, an interstitial defect has formed. This is possible because there is space between the atoms in the crystal lattice. While point defects are irregularities in a single site of a crystal lattice, line defects are irregularities in the plane of atoms. Dislocations are a type of linear defect associated with the misalignment of these planes. In the case of an edge dislocation, a half-plane of atoms is wedged between two planes of atoms. In the case of a screw dislocation, two planes of atoms are displaced, and a spiral array passes between them.[8]

In glasses, hardness appears to depend linearly on the number of topological constraints acting between the atoms of the network.[9] Consequently, the theory of hardness made it possible to predict hardness values ​​by composition.

Atomic planes split by an edge dislocation.

Dislocations provide a mechanism for sliding atomic planes and therefore a method of plastic or permanent deformation.[7] The planes of atoms can flip from one side of a dislocation to the other, allowing the dislocation to pass through the material and become permanently deformed. The movement allowed by these dislocations results in a decrease in the hardness of the material.

A way to suppress the motion of atomic planes and thus make them more complex involves the interaction of dislocations with each other and with interstitial atoms. When a dislocation intersects with a second dislocation, it can no longer pass through the crystal lattice. The intersection of dislocations creates a tether point and prevents the atomic planes from continuing to slide past each other.[10] A dislocation can also be pinned through interaction with interstitial atoms. If a dislocation comes into contact with two or more interstitial atoms, the sliding of the planes will again be disrupted. Interstitial atoms create anchor points or pinning points in the same way as intersecting dislocations.

By varying the presence of interstitial atoms and dislocation density, the hardness of a particular metal can be controlled. Although it may seem counterintuitive, as dislocation density increases, more intersections are created and therefore more attachment points. Likewise, the more interstitial atoms are added, the more pinning points that impede the movement of dislocations are formed. As a result, the more support points added, the harder the material will become.

Hardness of electroplated coatings

In the case of electroplated coatings, it should be noted that due to their small thickness, many methods (especially indentation methods) may not be suitable. The most common methods are Mohs and Vickers.

To measure hardness, a coating with a minimum thickness of 2 µm is required. If a smaller thickness is required, use GOST 9013-59, GOST 9012-59, GOST 22761-77

The measurement principle is the same. After applying the coating and drying it, the quality control department takes measurements and makes a decision - to ship the product or send it for recoating.

An important role here is played by both the electrolyte in which the coating is applied and the coating application mode (temperature, current density). For example, in one chrome plating electrolyte it is possible to obtain a chrome coating with a hardness from 500 to 1100 kgf/mm2.

If we talk about the electrolyte, the most important role is played by the quantity and quality of the brightening agents in it. A matte zinc coating will be much softer than a shiny one. Therefore, if you want a super-shiny coating, keep in mind that it will be hard, and there is a possibility of it cracking or peeling off at the slightest bending of the product.

further reading

• Chinn, R. L. (2009). "Hardness, Bearings and Rockwells". Modern materials and processes
  .
  167
  (10):29–31.
• Davis, J. R. (ed.). (2002). Surface hardening of steels: basics.
  Materials Park, Ohio: ASM International.
• Dieter, George E. (1989). Mechanical metallurgy.
  SI Metric adaptation. Maidenhead, UK: McGraw-Hill Education. ISBN 0-07-100406-8
• Malzbender, J (2003). “Comment on the definitions of hardness.” Journal of the European Ceramic Society
  .
  23
  (9): 9. Doi:10.1016/S0955-2219(02)00354-0.
• Revankar, G. (2003). "Introduction to Hardness Testing". Mechanical Testing and Evaluation
  , ASM Online Vol. 8.

Mohs scale

The Mohs hardness scale is relative and is used exclusively for minerals. Ten minerals were selected as reference minerals, which were arranged in increasing order of their hardness (in the photo diagram below). Accordingly, the scale has 10 points (from 1 to 10).

The mineralogical hardness scale was proposed by the German scientist Friedrich Mohs back in 1811. Nevertheless, it is still used in geology.

How to determine the hardness of a particular mineral on the Mohs scale? This can be done by carefully examining the scratch left by the sample. It is convenient to use a fingernail, a copper coin, a piece of glass or a steel knife.

So, if the tested mineral writes on paper without scratching it, then its hardness is equal to one. If a stone is easily scratched by a fingernail, its hardness is 2. Minerals that are easily scratched by a knife have three points. If you need to make some effort to leave a mark on the stone, then its hardness is 4 or 5. Minerals with a hardness of 6 or higher themselves leave scratches on the knife blade.