Strengthening Mechanisms of Materials - Strengthening Mechanisms in Metals - Transformation Hardening

Transformation Hardening

This method of hardening is used about steels.

High-strength steels generally fall into three basic categories, classified by the strengthening mechanism employed. 1- solid-solution-strengthened steels (rephos steels) 2- grain-refined steels or High strength low alloy steels (HSLA) 3- Transformation-hardened steels

Transformation-hardened steels are the third type of high-strength steels.These steels use predominately higher levels of C and Mn along with heat treatment to increase strength. The finished product will have a duplex microstructure of ferrite with varying levels of degenerate martensite. This allows for varying levels of strength. There are three basic types of transformation-hardened steels. These are dual-phase (DP), transformation-induced plasticity (TRIP),and martensitic steels.

The annealing process for dual -phase steels consists of first holding the steel in the a + gama temperature region for a set period of time. During that time C and Mn diffuse into the austenite leaving a ferrite of greater purity. The steel is then quenched so that the austenite is transformed into martensite, and the ferrite remains on cooling. The steel is then subjected to a temper cycle to allow some level of martensite decomposition. By controlling the amount of martensite in the steel, as well as the degree of temper, the strength level can be controlled. Depending on processing and chemistry, the strength level can range from 350 to 960 MPa.

TRIP steels also use C and Mn, along with heat treatment, in order to retain small amounts of austenite and bainite in a ferrite matrix. Thermal processing for TRIP steels again involves annealing the steel in the a + g region for a period of time sufficient to allow C and Mn to diffuse into austenite. The steel is then quenched to a point above the martensite start temperature and held there. This allows the formation of bainite, an austenite decomposition product. While at this temperature, more C is allowed to enrich the retained austenite. This, in turn, lowers the martensite start temperature to below room temperature. Upon final quenching a metastable austenite is retained in the predominately ferrite matrix along with small amounts of bainite (and other forms of decomposed austenite). This combination of microstructures has the added benefits of higher strengths and resistance to necking during forming. This offers great improvements in formability over other high-strength steels. Essentially, as the TRIP steel is being formed, it becomes much stronger. Tensile strengths of TRIP steels are in the range of 600-960 MPa.

Martensitic steels are also high in C and Mn. These are fully quenched to martensite during processing. The martensite structure is then tempered back to the appropriate strength level, thus adding toughness to the steel. Tensile strengths for these steels range as high as 1500 MPa.

Read more about this topic:  Strengthening Mechanisms Of Materials, Strengthening Mechanisms in Metals

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