Hardenability of Steels
Base hardenability of course increases with increase in carbon content for hypoeutectoid steels. But for hypereutectoid steels (above %carbon) hardenability. The maximum attainable hardness of any steel depends solely on carbon content . Also, the maximum hardness values that can be obtained with small test. And while there are steels that have up to 2 percent carbon content, they They have the least amount of chromium, offer high hardenability, the iron-carbon- steel relationship and its influences on welding and metal alloys.
With up to 1. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties. Low-alloy Steels When these steels are designed for welded applications, their carbon content is usually below 0. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.
These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them.
Metallurgy Matters: Carbon content, steel classifications, and alloy steels - The Fabricator
Preheating might also prove necessary. This can be determined by using the carbon equivalent formula, which we'll cover in a later issue. High-alloy Steels For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents.
Hardenability of Steels
The three basic types of stainless are: Austenitic Ferritic Martensitic Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone HAZ. Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.
Austenitic stainless steels offer excellent weldability, but austenite isn't stable at room temperature.
Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.
Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium.
And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It's important to note that austenitic alloys can't be hardened by heat treatment.
Boron can therefore only affect the hardenability of steels if it is in solution. This requires the addition of "gettering" elements such as aluminium and titanium to react preferentially with the oxygen and nitrogen in the steel. Chromium, molybdenum, manganese, silicon, nickel, vanadium The elements Cr, Mo, Mn, Si, Ni and V all retard the phase transformation from austenite to ferrite and pearlite. The most commonly used elements are Cr, Mo and Mn. The retardation is due to the need for redistribution of the alloying elements during the diffusional phase transformation from austenite to ferrite and pearlite.
The solubility of the elements varies between the different phases, and the interface between the growing phase cannot move without diffusion of the slowly moving elements. There are quite complex interactions between the different elements, which also affect the temperatures of the phase transformation and the resultant microstructure.
- Metallurgy Matters: Carbon content, steel classifications, and alloy steels
Steel compositions are sometimes described in terms of a carbon equivalent which describes the magnitude of the effect of all of the elements on hardenability. Grain size Increasing the austenite grain size increases the hardenability of steels.
The products of the transformation, whether ferrite, pearlite or bainite, are partly determined from isothermal diagrams, and can be confirmed by metallographic examination. This series of results will give rise to an austenite-pearlite boundary on the diagram and likewise lines showing the onset of the bainite transformation can be constructed.
A schematic diagram is shown in Figure 3. The diagram is best used by superimposing a transparent overlay sheet with the same scales and having lines representing various cooling rates drawn on it. The phases produced at a chosen cooling rate are those which the superimposed line intersects on the continuous cooling diagram.
In this example, it should be noted that the centre cooling curve intersects the bainite region and consequently some bainite would be expected at the core of the bar after quenching in oil.
TTT diagram of a molybdenum steel 0. Relation between cooling curves for the surface and core of an oil-quenched 95 mm diameter bar and the microstructure Hardenability Testing The rate at which austenite decomposes to form ferrite, pearlite and bainite is dependent on the composition of the steel, as well as on other factors such as the austenite grain size, and the degree of homogeneity in the distribution of the alloying elements.
It is extremely difficult to predict hardenability entirely on basic principles, and reliance is placed on one of several practical tests, which allow the hardenability of any steel to be readily determined: The Grossman test The Jominy end quench test Effect of Grain Size and Chemical Composition on Hardenability The two most important variables which influence hardenability are grain size and composition.What Is The Hardenability Of Steel?
The hardenability increases with increasing austenite grain size, because the grain boundary area is decreasing. This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased.
Likewise, most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability.
However, quantitative assessment of these effects is needed. There are a bewildering number of steels, the compositions of which are usually complex and defined in most cases by specifications, which give ranges of concentration of the important alloying elements, together with the upper limits of impurity elements such as sulfur and phosphorus.