The hardness of metals and alloys. What does it affect? How do you increase the hardness of a given material?
Metal remains the most used and widespread material to this day, despite the abundance of new materials produced using up-to-date technologies. All manufacturing industries keep using different types of metals and alloys. Construction, electric power production, mechanical engineering, defense, and medical sectors – all areas of life are dependent on that material in different grades. Naturally, the matter of quality indicators becomes of paramount importance.
The metal hardness indicator is on top of them. According to the definition, it means that a material can resist elastic deformation, plastic deformation, and (or) destruction of surface layers induced by a more rigid physical object with its resistance to permanent damage after the contact (impact) (the intender of the hardness tester, for example).
For many objects under testing, it becomes hard or almost impossible to produce samples that can be subjected to thorough examination in a laboratory. Hardness becomes the only available feature that portable hardness testers can measure. Measuring metals' hardness makes it possible to study the object's construction features, its usage in all details, evaluate the depreciation rate along different periods, and how different temperatures may affect it, among many other factors.
This simple indicator will demonstrate how long a bridge will last, what pipeline should be constructed, how fast a machine-tool will be erased, how a given specific tool blank should be sharpened, how safe a car is, or how long dental implants are going to last. And the list can go on. As you can see, metal's hardness is directly linked with all the most essential characteristics of the final manufactured product or its constructional features.
What characteristics of the product are affected by hardness?
Each area needs its indicators and criteria.
For example, let us consider wear-resistance, which is the susceptibility of metal to abrasion, surface destruction, and size changes during usage under certain conditions. It is crucial in every area of use of this material.
From construction kits for kids to a new viaduct, from a surgical needle to a communication tower, from a gas pipe to an engagement ring – it is impossible to find an area where this feature would not be considered essential or paramount. Naturally, the higher the wear resistance, the longer the product will last, and the higher is its cost.
The next thing directly affected by hardness is the processing of the particular metal or alloy and the type of such processing.
There are several major types of metal fabrication processes:
- mechanical working
- heat treating
- using pressure
- electric discharge
- chemical technologies.
Of course, when choosing a method, several criteria must be taken into account (in short, the initial properties of the metal and the desired result), but the initial material's hardness is one of the fundamental ones in this matter.
The hardness of metals also affects the resistance to pressure and other forces, which is important, for example, for shafts or bearings affected by centrifugal and frictional forces.
The amount of hardness of the material determines the possibility of using the product as a tool for working with other metal or non-metal products. Here we are dealing with high carbon steel, which has high carbon content (from 0,7% and higher). It is used to make various tools for both industrial and home use: drills, milling cutters, hammers, pliers, files, surgical scissors, scalpels, among other things.
The natural conclusion from all of the above should be recognizing the great importance of the indicator of the hardness of metals and how to increase it.
!!! It is important to note that requirements are set up for each product individually. Thus, there is no such a concept as "higher the hardness, the better." For example – pieces with high hardness rates are more difficult to process, and after processing, they become quite brittle. In case we consider a knife, the harder the knife material is, the higher the sharpness will be. However, sharpening may become tricky, and the blade may start to crumble and break easily after frequent use.
The most common ways to increase hardness are:
- heat treatment (different types of quench hardening processes, recrystallization)
- chemical and heat treatment (alloying with chromium and other elements, Aluminium coating, etc.)
- mechanical (bending, artificial aging, rolling, etc.)
Each way of increasing hardness has its effects.
Steel hardening is one of the most ancient ways to improve metal quality. It can be anything from cold weapons to agriculture tools. The hardening process increases metal hardness by subjecting it to critical heating and subsequent quick cooling.
After passing through such a hardening process, the carbon steels can be used to produce parts that must be extremely tough (bushings, shafts, gears, etc.). The whole process is technologically sophisticated, and it is far from being costly, making its advantages obvious.
Metal alloying using such catalysts as tin (Sn), nitrogen (N), and lead (Pb) eventually makes it possible to produce large parts that are supposed to be subjected to intense loads. They are large-sized springs and spring mechanisms.
Adding chromium (Cr) to the alloy increases metal strength and resistance to corrosion. The stainless steel must contain at least 13% chromium.
Another widely used way to boost the quality of metal and alloys is work hardening (or strain hardening), which consists of plastic deformation leading to a higher strength of the material. As a result of this process, the hardness of the upper layers of the material multiplies. To alloys resistant to corrosion, such as alloys with chromium and nickel, this is the only way to increase hardness.
The ways metals and alloys are used
Aluminum and alloys with this metal are among the most common materials. This metal is used to manufacture innumerable household objects, mirrors, parts for small arms, and even the fuel for space rockets.
Thanks to aluminum's low specific gravity, the alloys of this metal can be used to manufacture aircraft hulls and different machines.
Copper is often added to the alloys to improve the quality of hardware and manufacture different wires and pipes.
Bolts, screws, screws, anchors, among others, are mainly made of brass and bronze. Clamps, wing nuts, and other fixing elements are often produced from different alloys and structural steel. Stainless steel is the best when it comes to the high risk of corrosion. Cast iron is still effectively used in the manufacture of valves and metal production.
Different products made from metals and alloys are all around us. Buildings, roads, bridges, cars, public transport - we use metal structures every day without even noticing it. That's why it is crucial to be confident in the materials' quality and keep the hardness under the radar.