Info

Oil Quenched

Oil Quenched

Tempering Temperature (° F)

Fig. 5.16. Effects of Tempering Temperature on 4340 Steel14

Tempering Temperature (° F)

Fig. 5.16. Effects of Tempering Temperature on 4340 Steel14

Fig. 5.17. Heat Treatment for Medium Carbon Low Alloy Steels

The procedure for hardening is illustrated in Fig. 5.17 and consists of the following:15

(1) Austenitizing. During austenization, the steel is heated into the austen-ite (y) field and held for a sufficient period of time to dissolve many of the carbides and put them into solution. As shown in Fig. 5.18, the

Composition (Wt% C)

Fig. 5.18. Steel Heat Treating Ranges15.

Composition (Wt% C)

Fig. 5.18. Steel Heat Treating Ranges15.

temperature required for austenization is a function of the carbon content; with increasing carbon contents the temperature decreases along the A3 line to a minimum value A1 at the eutectoid composition (0.8% carbon) and then increases again along the ACm line. The first stage in the formation of austenite is the nucleation and growth of austenite from pearlite (ferrite + Fe3C). Even after the complete disappearance of pearlite, some carbides will remain in the austenite. To minimize the time for austeniza-tion, the temperature used is about 100° F above the minimum temperature for 100% austenite, and the time is about one hour per inch of thickness. However, it is also important to keep the austenization temperature as low as possible to reduce the tendency toward cracking and distortion, minimize oxidation and decarburization, and minimize grain growth. The addition of alloying elements, such as manganese and nickel, helps to reduce the temperature necessary for austenite formation.

(2) Quenching. While the FCC austenite that forms during austenization is capable of dissolving as much as 2% carbon, only a small fraction of carbon can be retained in the lower temperature BCC ferrite. If the steel is slowly cooled from the austenization temperature, carbon atoms are rejected as the FCC austenite transforms to the BCC ferrite, and alternating layers of ferrite and cementite form pearlite through a nucleation and growth process. However, if the steel is rapidly cooled from the austenization temperature (quenched), the carbon does not have time to diffuse out of the austenite structure as it transforms to the BCC ferrite, and the BCC structure becomes distorted into a BCT tetragonal structure called martensite. The objective of the quenching process is to cool at a sufficient rate to form martensite. The distortion of the BCT structure results in high strength and hardness of the quenched steel. As previously shown in Fig. 5.17, the steel must be cooled past the nose of the isothermal transformation diagram to form 100% martensite. Martensite does not form until it reaches the martensite start temperature (Ms) and is complete after it is cooled below the martensite finish temperature (Mf). The addition of alloying elements increases the hardenability of steels by moving the nose of the isothermal transformation diagram to the right, allowing slower cooling rates for alloy steels to form martensite. However, alloying elements also often depress the Ms and Mf temperatures so that some highly alloyed steels must be cooled to below room temperature to obtain fully martensitic structures. Medium carbon low alloy steels are quenched in either water or oil to produce adequate cooling rates. While water quenching produces the fastest cooling rates, it also produces the highest residual stresses and can often cause warpage and distortion; therefore, higher alloy grades are often used so that oil quenching can be used.

(3) Tempering. While as-quenched steel is extremely hard and strong, it is also very brittle. Tempering, in which the steel is reheated to an intermediate temperature, is used to increase the ductility and toughness with some loss of strength and hardness. During tempering, the highly strained BCT structure starts losing carbon to transformation products which reduces lattice strains producing an increase in ductility and a reduction in strength. One of the advantages of medium carbon low alloy steels is the large range of strength values that can be obtained by varying the tempering temperature, as previously shown in Fig. 5.16 for 4340 steel.

Medium carbon low alloy steels can be susceptible to two types of temper embrittlement during heat treating: (1) one-step temper embrittlement and (2) two-step temper embrittlement.2 Commercial heat treatments and compositional controls to eliminate unwanted impurities are designed to avoid these embrittle-ment mechanisms.

One-step temper embrittlement. This type of embrittlement, also known as 660° F embrittlement, occurs in high strength low alloy steels that have quenched and tempered martensitic structures. When the alloy is austenitized, quenched, and then tempered for a short time (about 1 h) between 480 and 660° F, it can cause a decrease in notched toughness and impact strength. The failure mode is intergranular and is thought to be due to the impurity elements phosphorous, nitrogen, and possibly sulfur, since tests on high purity alloys do not exhibit embrittlement. Manganese may play an indirect role by helping the impurity elements segregate to the grain boundaries.

Two-step temper embrittlement. This type of embrittlement causes a decrease in notch toughness when tempered alloy steels are isothermally aged in the temperature range of 700-1040° F, or are slowly cooled after tempering. Two-step embrittle-ment results in intergranular failure modes and is also attributed to the presence of impurities that segregate to the grain boundaries. The ductile-to-brittle transition is directly dependent on the grain boundary concentration of impurities. The relative effect of these impurities has been found to be tin > antimony > phosphorous. Alloying elements can also cosegregate to the grain boundaries along with the impurities, i.e. nickel cosegregates with antimony. The rate and amount of impurity segregation, and hence the resultant intergranular embrittlement, depends on the total composition of the system. Nickel, chromium, and manganese increase two-step temper embrittlement caused by antimony, tin, phosphorous, or arsenic. Additions of molybdenum retard temper embrittlement since molybdenum inhibits the segregation of impurities, i.e. molybdenum readily precipitates as phosphides in the matrix and inhibits segregation.

Was this article helpful?

0 0

Post a comment