Hardenability and hardenability are two performance indexes to characterize the capacity of steel to accept quenching, and they are also important basis for material selection and use
1.Concept Of Hardenability And Hardenability
Hardening is the ability of steel to achieve the highest hardness after hardening to martensite under ideal conditions. The main factor that determines the hardness of steel is the carbon content of the steel, more specifically the carbon content of the steel that is solidly dissolved in austenite during quenching heating. The higher the carbon content, the higher the hardness of the steel. The alloying elements in steel have little effect on hardenability, but have great effect on hardenability.
Hardenability means steel under certain conditions quenching to obtain the ability to quench the depth of the layer, mainly affected by the carbon content and alloying elements in austenite, refers to under specified conditions, determine the steel hardening depth and hardness and hardness distribution characteristics, steel hardenability is good and bad, commonly used to express the depth of the quench layer, the greater the depth of the quench layer, the better the hardenability of steel, The hardenability of steel mainly depends on its chemical composition. Good hardenability of steel can make the whole section of steel obtain uniform mechanical properties and the quenching medium with small quenching stress can be selected to reduce deformation and cracking
In addition, because hardenability and hardenability are two concepts, so after quenching hardness of steel, not necessarily high hardenability; Steels with low hardness may also have high hardenability.
2. Hardenability Evaluation Method
The current universally accepted method in the heat treatment industry is HP-IVF method of Swedish IVF Company, which was pioneered by Dr.Soren Segerberg of Swedish IVF
2.1 Calculation formula for ISO9950 quenching oil hardening:
HP – IVF Tvp (oil) = 91.5 + 1.34 + 10.88 CR550a – 3.85 Tcp
Note: Tvp is the transition temperature (characteristic temperature) of the vapor film to the boiling stage.
CR550a is the average cooling rate between 600-500℃ (which can be obtained directly by IVF software)
Tcp is the transition temperature from boiling to convection
(intended for unalloyed steels)
2.2 Calculation formula for quenching liquid hardening according to ASTM standard:
HP – IVF (polymer) = 3.54 + 12.3 CR325b CR550-168
Note: CR550 represents the CR value of 550 degrees Celsius
So CR325b is CR at 325 degrees Celsius
(intended for unalloyed steels)
3. Factors Affecting Hardenability
The hardenability of steel depends on the stability of austenite. All the factors that can improve the stability of supercooled austenite, shift the C curve to the right, and thus reduce the critical cooling rate can improve the hardenability of high steel. The stability of austenite depends mainly on its chemical composition, grain size and composition uniformity, which are related to the chemical composition and heating conditions of the steel.
4. Determination Method Of Hardenability
There are many ways to determine the hardenability of steel, including critical diameter test and end quenching test.
(1) Determination of Critical diameter The maximum diameter of the steel after quenching in a certain medium, when the core is fully martensitic or 50% martensitic tissue, is called the critical diameter and is represented by Dc. The critical diameter measurement method is to make a series of round rods with different diameters. After quenching, the hardness U curve distributed along the diameter on the section of each sample is determined respectively, from which the round rod whose center is exactly half martensite structure is found, and the diameter of the round rod is the critical diameter. The larger the critical diameter, the higher the hardenability of the steel.
(2) End quenching test method End quenching test method is to use a standard size end quenching sample (diameter of 25mm×100mm), after austenitizing, on the special equipment to cool its end face with water, cooling along the axis to measure the hardness – distance from the water cooling curve. End quenching test is one of the ways to determine the hardenability of steel. Its advantages are simple operation and wide application.
