Info

0.25 Mn, 0.25 Si, 1.50-1.80 Ti

1333° F for some period of time and then quenched to room temperature, it does not convert to the normal BCC structure. Instead, it converts to a body centered tetragonal (BCT) structure called martensite. This transformation is shown in the isothermal transformation diagram for 4340 steel in Fig. 5.3. The BCT martensite structure is essentially a BCC structure distorted by interstitial carbon atoms into a tetragonal structure (Fig. 5.4). The distortion severely strains the crystalline lattice and dramatically increases the strength and hardness.

Weight % Carbon

S Ferrite - BCC Y Austenite - FCC a Ferrite - BCC Fe3C - Cementite

Weight % Carbon

S Ferrite - BCC Y Austenite - FCC a Ferrite - BCC Fe3C - Cementite

Fig. 5.2. Iron-Carbon Phase Diagram

Unfortunately, it also makes the steel extremely brittle; therefore, the steel is reheated, or tempered, at intermediate temperatures as shown in the schematic of Fig. 5.5 to restore some ductility and toughness, although the strength also decreases as the tempering temperature is increased.

A key variable in heat treating alloy steels is the cooling rate during quenching. The quench rate that will provide the desired hardness for a given thickness is determined primarily by alloying additions. Some alloys require a water quench, others an oil quench, and some are so highly alloyed that they can be air cooled to room temperature to form a martensitic structure. Since the cooling rate is also dependent on section size, the quench may have to be changed as the thickness increases, for example, a steel that could be through-hardened with an oil quench at a thickness of 1/2 in. may have to be water quenched when the thickness is increased to an inch.

Fig. 5.3. Isothermal Transformation Diagram for 4340 Steel

While some steels are hardened by the conversion of austenite to martensite through a quench and temper process, others, such as the maraging and precipitation hardening steels, are strengthened by precipitation hardening. The maraging steels, with nominal nickel contents of 18% and carbon contents of only 0.03%, will form martensite on air cooling from the austenitizing temperature. Even very slow cooling of heavy sections produces a fully martensitic structure. However, this low carbon martensite is not the high strength martensite that forms in the higher carbon alloy steels. The influence of carbon on the strength and hardness of steel is shown in Fig. 5.6. The low carbon martensite that is formed is a tough and ductile iron-nickel martensite. The strength in the maraging steels results during age hardening at 850-950° F to form precipitates of Ni3Mo and Ni3Ti.2 Since the carbon content is extremely low, maraging steels

Iron

Iron

Fig. 5.5. Heat Treatment for Medium Carbon Low Alloy Steels

Fig. 5.5. Heat Treatment for Medium Carbon Low Alloy Steels are characterized by a combination of high strength, ductility, and excellent toughness.

The precipitation hardening (PH) stainless steels are classified as either semi-austenitic or martensitic. The semiaustenitic grades contain an austenitic structure in the annealed or solution treated condition. After fabrication operations are complete, they can be transformed to martensite by a simple conditioning treatment followed by precipitation hardening. The conditioning treatment consists of heating the alloy to a high enough temperature to remove carbon from solid

Carbon Content (%)

Fig. 5.6. Effect of Carbon Content on Hardness of Steel

Carbon Content (%)

Fig. 5.6. Effect of Carbon Content on Hardness of Steel solution and precipitate it in the form of chromium carbide (Cr23C6).3 Removing carbon from solid solution makes the austenite unstable, and it starts transforming to martensite on cooling to the martensite start (Ms) temperature. Depending on the specific alloy composition and the conditioning temperature, some of these grades must be cooled to subzero temperatures to cause the conversion from austenite to martensite. The final step is precipitation hardening in which the alloy is reheated to 900-1200° F. During this aging treatment, aluminum in the martensite combines with nickel to produce precipitates of NiAl and Ni3Al.3 The martensitic grades of precipitation hardening stainless steels are essentially martensite after solution treating. They are therefore harder and stronger than the semiaustenitic grades before precipitation hardening. Since they are already martensitic, a conditioning treatment is not required; they only require the final precipitation hardening treatment to attain maximum strength and hardness.

Was this article helpful?

0 0

Post a comment