Hardness and Hardenability: Factors and Tests
Hardness and Hardenability: just a picture
What is the definition of hardness? What is the definition of hardenability? In most cases, many get confused between the two terms, hardness and hardenability. These two terms are used to explain the mechanical properties of metals. Strain-hardening enhances the hardness of metals. Hardness is a property of metal which is the ability of metal to resist indentation. Hardenability as another property of metal determines the depth at which any material can be hardened. The higher the hardenability of any metal, the tougher the metallic material in question.
There are many properties that determine the hardenability of metals. These properties affect the hardenability of metals in one way or the other. The properties that affect the hardenability of metals are amount of alloying elements, austenitizing temperature and time, nature of coolant, criterion of hardenability, grain size, and the amount of carbon content.
An Illustration on Hardness
Hardness and Hardenability-Know the Differences
Hardness is the ability of any metal to resist indentation. The word, indentation, can be used in the place of notch. So, hardness in the other words can be defined as the ability of any metal to resist notch. It is the resistance to indentation or notch. The resistance to indentation on any metal can either take place at the surface or edge of the metallic material. Hardness is a mechanical and physical property of metallic objects. In metals, when the hardness is much, the toughness and strength increases. The hardness of materials is tested before they are used in any place to detect their strength. This analysis is done before alloys are used in constructions of buildings and bridges.
Hardness varies inversely with ductility of metallic materials. Ductility is the property of metals which enables the metals to be drawn into wire. The softer a material, the more ductile the material becomes. Ductility and brittleness are like “cat and dog”. In the other words, ductility is opposite of brittleness. The ductility of polymers can be enhanced by the use of plasticizers and this in turn reduces the hardness of polymers.
Hardness of materials can be reduced by grain growth and annealing process. When the grains that make up a metallic material, say brass, are grown, the sizes of the grains become increased. This increase in the size of the grains reduces the number of the grain boundaries in that structure. Because of the reduced number of the grain boundaries, the metal becomes less hard and more malleable. At that point, the material becomes softer than before.
Annealing of metals is another way of reducing the hardness of metals. Annealing is a heat treatment process where metals are heated to elevated temperature for a long period of time and slowly cooled. It reduces strain-hardening of metallic materials. Annealing increases the softness and ductility in metals and hence decreases the metals hardness.
The hardness of quenched Steel generally refers to the hardness of 100% martensitic structure. It is mainly a function of Carbon content even in most alloy Steels. Quenching in metallurgical and mechanical study can be defined as rapid cooling of any metal in any medium, which can be water, oil, air or even brine.
Hardenability is the depth to which any metal can be hardened. It is the depth of hardening on quenching any metallic material. The depth of hardness penetration (on hardening) is characteristic property of steel, and is called hardenability. The susceptibility to hardening by rapid cooling is defined as a property of steel which determines the depth and distribution of hardness produced by quenching, or as the capacity of steel to develop a desired degree of hardness is usually measured in terms of depth of penetration.
Factors Affecting the Hardenability of Metals (Steel)
There are certain properties that determine the hardenability of any metal. In a nutshell, hardenability is dependent on the following:
- Amount of alloying element
- Austenitising temperature and time
- Nature of the coolant
- Criterion of hardenability
- Grain size of the quenched metal
- The amount of the carbon content in the steel
Most of the discussion on this topic will be based on steel as one of the metals that play important role in many metallurgical industries.
Amount of alloying element
An alloy is a mixture of two or more metals to improve the mechanical properties of metals. The proportion of alloying element used determines the nature of the final product. Metal alloying elements slow the reaction rate of pearlite and ferrite. The more alloying elements in a parent metal, the less the reaction rate in that metal. Metallic alloying elements increase the stability of austenite resulting in shifting the CCT curve (Continuous Cooling Transformation Curve) towards the right. An austenite is an interstitial solid solution of carbon in gamma-iron. It is Face Centre Cubic in structure (FCC).
The only metal alloying element that does not slow down the reaction of ferrite and pearlite is Cobalt. Most alloying elements increase the hardenability of metals but there is an exception. The exception is found in ferrite. Cobalt decreases the hardenability of steel instead of increase when compared with other alloying element. In steel, Cobalt increases the nucleation and growth of Pearlite and shift the CCT towards the left.
Some alloying elements that increase hardenability are Manganese, Chromium, Silicon, and Molybdenum. Whenever any of these elements are used in appropriate proportion and on right base metals, the hardenability is increased. The addition of alloying element like Boron in limited amount (0.003 to 0.005%) also increases the hardenability of steel to some extent.
Austenitising temperature and time
Austenitising temperature and time play important role in hardenability of steel. Also, the higher the austenitising temperature and time the higher the hardenability of steels. When steel is heated to a very high temperature and time, the austenite grains become coarsen. The coarseness of the grains of the steel increases the hardenability of metallic materials.
This is another important factor that determines the hardenability of metals. The larger the grains size of any metal, higher its hardenability. In the order words, as the grain size of metals increase, their hardenability tends to increase. As hardenability was defined as the depth of hardness penetration, what it means is that when a metal is made of coarse grains instead of fine ones, hardness can easily penetrate into the metal. If the grain sizes that make particular steel are small in size, the hardenability of that steel will be low. This is so because there are lots of grain boundaries at that point which cannot easily give room to hardness penetration.
When austenite grain size is large, the grain boundary area decreases. Because of this, the nucleation sites of the austenite (for steel) become reduced in number and thus results to increase in hardenability. This means of hardenability increase is sometimes ignored because of its negative effect that includes brittleness and susceptibility to crack.
Nature of coolant
The kind coolant used during hardening of metal by quenching or any process affects the hardenability of metals. A steel transforms from austenite to martensite if it’s critical cooling rate is more than, or equal to the critical rate. When the cooling rate of steel is greater than the critical cooling rate, the hardenability of the steel increases.
The hardness level that is obtained when steel is cooled in brine is not the same when the same steel is cooled in oil. When the rate of giving out heat is high, the hardenability of the metal is increased. For instance, cooling in brine evolves heat more easily that cooling in oil and this results to higher hardenability when the metal is cooled with brine.
The amount of carbon in steel determines the maximum attainable hardness on quenching. Carbon content increases hardenability in steel. The reason is because carbon stabilizes austenite. The stability makes Continuous Cooling Transformation curve to shift to the right. Note that whenever the CCT curve shifts to the right, hardenability increases but when if shifts towards the left, hardenability decreases. It is very necessary to know that the hardenability of steel is dependent on the elements that make it up. For instance, commercial steel always have manganese and other elements by the process of making steel economically, which increases their hardness. Carbon fixes the maximum attainable hardness on quenching.
Criterion for hardenability
Universally agreed criterion of hardenability is the depth which contains 50% martensite and 50% other products. Other criterion can be 90% martensite and 10% other products. The 50% martensite and 50% other products have been used in production of most alloy steels. If any other criterion is used apart from that which is universally accepted, it is indicated on the product. The criterion for hardening of metals is necessary for metallurgical engineering applications. The 50% martensite can be from the outer surface of the metal to the inner part while other products, which can be soft core, occupy the remaining inner part.
Hardenability test is a test that determines how materials are hardened. Hardenability tests determine the level of hardness penetration (hardenability) in metals. The three major hardenability tests are:
- Grossman's method;
- Jominy end-quench method; and
- Hot brine (H-B) test.
Grossman performed his own hardenability test based on critical diameter. It is a time-consuming method of determining the hardenability of metals. He stated that steel sections with similar half cooling time have similar micro-structure and similar hardness as well. He stated that as the diameter of a cylindrical steel increases, its hardenability decreases, and as the diameter decreases the hardenability increases in return.
The critical diameter hardening is dependent on the quenching medium applied. The more easily heat is relieved from the steel, more hardenability the material possesses. For instance, the hardenability of steel quenched with water is more than when compared the same material quenched in oil.
Jominy end-quench method
Jominy method is more practically applied than that of Grossman, which is based on ideal diameter, and also hot brine test method. Grossman's method is labourous, expensive, and difficult. It is used for determining the hardenability of steel up to 1 to 6 inches. Jominy end-quench method is simple and easy. It involves the use of oversized bar which is finally machined to the right size.
When the bar is heated to austenitising temperature, it is left for some minutes, like 30 minutes, and then introduced into a quenching medium like water. In this method, one end of the bar is quenched. The hardness is obtained by grinding two parallel surfaces and then determined.
Hot Brine Test
It is applied where Jominy end-quench test cannot play good part. This method is used for quenching steel of diameter of less than one inch. The method of testing hardenability was proposed by a man called Grange.
This topic is based on hardness and hardenability as properties of metallic materials which apply in everyday life. Clear explanations were given on the two principal words and discussed are also factors affecting hardenability of metals.
Factors that affect hardenability of metals are grain size, austenitising temperature and time, nature of coolants, criterion of hardenability, amount of carbon contents, and the amount of alloying elements.
Hardenabilty tests which include Grossman’s method, Jominy end-quench and hot brine test were discussed. Among all these three methods, the best and most used is the Jominy end-quench method. Jominy end-quench method is easy and simple method of determining the hardenability of materials.
- Materials Science and Engineering: An Introduction by William D. Callister, Jr. and David G. Rethwisch
- Heat Treatment of Metals by Virjendra Singh
- Testing of Metals by Tata McGraw-Hill Education