Engineering Materials: Understanding Hardenability and its Impact on Steel Properties - Pr, Lab Reports of Mechanical Engineering

Download Engineering Materials: Understanding Hardenability and its Impact on Steel Properties - Pr and more Lab Reports Mechanical Engineering in PDF only on Docsity! 1 Southern Polytechnic State University MET3132: Engineering Materials Laboratory Dr. Atiqullah HARDENABILITY TEST Adapted from various sources Hardness and Heat Treatment • Ability of a material to resist permanent surface deformation. • Type of Tests: Brinell, Rock..., ?, ?, ? • Phase Transformation diagram- shows effect of composition and temperature on microstructure • Heat Treatment – to increase (decrease) hardness, toughness, relieve stress, ductility,... • Quenching – Heating up to AUSTENITE region of phase diagram and rapidly cooling to a low temperature ( by plunging in a liquid)- ‘freezes’ the microstructure of high temperature at lower temperature. • Quenching Media: water, oil, salt, cold gas – difference of ? • Quenching causes ‘locking up’ of microstructure and make it difficult to cause local deformation- HARD. • Usually quenching is followed by reheating to a desirable temperature for specific time – achieves other heat treatments through transformation of microstructure. 2 Hardenability • It is the ability of a steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface, when rapidly cooled under a given condition. • For example, a steel of a high hardenability can transform to a high fraction of martensite to depths of several millimeters under relatively slow cooling, such as an oil quench, whereas a steel of low hardenability may only form a high fraction of martensite to a depth of less than a millimeter, even under rapid cooling such as a water quench. • Hardenability therefore describes the capacity of the steel to harden in depth under a given set of conditions. Why hardening ? • Steels with high hardenability are needed for large high strength components, such as large extruder screws for injection molding of polymers, pistons for rock breakers, mine shaft supports, aircraft undercarriages, and also for small high precision components such as die-casting molds, drills and presses for stamping coins. • High hardenability allows slower quenches to be used (e.g. oil quench), which reduces the distortion and residual stress from thermal gradients. • Steels with low hardenability may be used for smaller components, such as chisels and shears, or for surface hardened components such as gears. 5 Effects of alloying and microstructure Jominy end quench test can be used to demonstrate the effects of microstructure and alloying variables on the hardenability of steels. These include alloying elements and grain size. Main alloying elements which affect hardenability are carbon, boron and a group of elements including Cr, Mn, Mo, Si and Ni. Increasing the carbon content increases the hardness of steels up to about 0.6wt%. Carbon also increases the hardenability of steels by retarding the formation of pearlite and ferrite. Boron is a very potent alloying/catalytic element, typically requiring 0.002 to 0.003wt% to have an equivalent effect as 0.5wt% Mo. Effects (contd.) The effect of boron is also independent of the amount of boron, provided sufficient is added, and the effect of boron is greatest at lower carbon contents. • 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. • Increasing the austenite grain size therefore decreases the available nucleation sites, which retards the rate of the phase transformation. That promotes hardenability. 6 Typical Hardness Test Data Which material has higher hardenability? How? Red Red curve goes up very rapidly near the quench end. Jominy Number J5 = 35 ? 5 mm ditance fron surface Which material has higher hardness? Blue Summary • The Jominy end quench test is the standard method for measuring the hardenability of steels. This describes the ability of the steel to be hardened in depth by quenching. • The hardenability depends on the alloy composition of the steel, • Hardenability can also be affected by prior processing, such as the austenitization temperature. • Knowledge of the hardenability of steels is necessary in order to – select the appropriate combination of alloy and heat treatment for components of different size, – to minimize thermal stresses and distortion. 7 What did you learn today?