Fabrication of Medium Carbon Low Alloy Steels

Forging7 is often selected for critical components because it creates a grain flow (Fig. 5.10) that increases ductility, impact strength, and fatigue strength. Forging breaks up casting segregation, reduces the as-cast grain structure, heals porosity, and helps to homogenize the structure. The improvement in transverse ductility as a result of forging for 4340 is shown in Fig. 5.11.

Medium carbon low alloy steels are only slightly more difficult to forge than carbon steels. They are supplied from the mill in either the annealed or the normalized and tempered condition. The selection of the forging temperature is based on carbon content, alloy composition, temperature range for optimum plasticity, and the amount of reduction required to forge the workpiece. In general, the forging temperature decreases with increases in carbon content and alloying elements. Maximum recommended forging temperatures are generally about 50° F lower than those used for plain carbon steels of the same carbon content. Medium carbon

Fig. 5.10. Etched 4140 Forging Showing Grain Flow7
0 10 20 30

Final Cross-Sectional Area

Forging Ratio = -

Original Cross-Sectional Area

Fig. 5.11. Influence of Forging Ratio on 4340 Steel8

low alloy steels are hot forged at temperatures ranging from 1950 to 2250° F. To avoid stress cracks that can result from air cooling, the forged part should be slowly cooled in a furnace or embedded in an insulating material. The section thickness, complexity, and size are often limited by the cooling that occurs when the part comes in contact with the cold dies; therefore, hammer forging with its short contact times is often used for forging intricate shapes. However, large landing gear components (Fig. 5.12) are usually forged in hydraulic presses due to their large sizes and the slow controlled strain rates that can be achieved in hydraulic presses. For steel forgings that are to be heat treated above a tensile strength of 150 ksi, the normal practice is to normalize before heat treatment to produce a uniform grain size and minimize internal residual stresses.

Medium carbon low alloy steels are usually formed in the annealed condition.9 Their formability depends mainly on the carbon content and is generally slightly less than for unalloyed steels of the same carbon content. They can be cut, sheared, punched, and cold formed in the annealed condition and then heat treated to the desired hardness. Because of their high strength and limited ductility, forming operations are not conducted in the quenched and tempered condition.

During machining, the higher hardness of these steels requires lower speeds and feeds than those used for the plain carbon steels. The machinability ratings10 of 4130, 4140, 4340, and 300M compared to cold rolled 1212 steel (100%) are

Fig. 5.12. Large Landing Gear Forging Source: Schultz, Steel

70, 65, 50, and 50% respectively, indicating that these materials are considerably harder to machine and require lower speeds and feeds. To improve machinabil-ity, medium carbon low alloy steels are normalized at 1600-1700° F and then tempered at 1200-1250° F prior to machining to produce a partially spherodized microstructure. Highly alloyed steels should be machined before hardening to martensite. If it is necessary to conduct machining operations after hardening, then a two-step process should be used in which the majority of the machining is done before heat treatment, and the machining conducted after hardening is essentially a light finish machining to provide dimensional accuracy. Finish machining, because of the relatively high hardness of the material, necessitates the use of sharp, well-designed carbide cutting tools with proper feeds, speeds, and a generous supply of coolant.

Due to their extreme strength and hardness, high strength steels are often finished by grinding to provide precise dimensions, remove any nicks or scratches, and provide smooth surfaces. However, great care must be taken when grinding these ultrahigh strength steels. Improper or abusive grinding can result in grinding burns in which the surface is heated above the austenitizing temperature and the austenite formed converts to untempered martensite on cooling, as shown in Fig. 5.13. This untempered martensitic surface layer is brittle and susceptible to forming a network of fine cracks that can reduce the fatigue strength by as much as 30%. Even if the grinding temperature does not produce austenite, it can result in overtempered martensite on the surface that is lower in hardness and strength.

Medium carbon low alloy grades are welded or brazed by a number of techniques.11 Alloy welding rods, comparable in strength to the base metal, are used and moderate preheating (200-600° F) is usually necessary. At higher carbon levels, higher preheating temperatures and post-weld stress relieving are often required. To avoid brittleness and cracking, preheating and interpass heating are used, and complex structures should be stress relieved or hardened and tempered immediately after welding. Since the weld joint cannot usually develop the high strength levels required in the as-welded or stress relieved condition, medium carbon low alloy steels are usually reaustenitized and then quenched and tempered after welding. The best condition for welding is either the normalized or the annealed condition.

Typical welding processes include inert gas tungsten arc, shielded metal arc, inert gas metal arc, and, in some instances, electron beam welding. The 4340 is weldable by a number of methods including both gas and arc welding. Welding rods of the same composition should be used. Since 4340 develops considerable hardness on air cooling, the welded parts should be either annealed or normalized and tempered shortly after welding. The medium carbon low alloy steels are also susceptible to hydrogen-induced cracking; therefore, every effort must be exerted to minimize any possible absorption of hydrogen gas. For some steels, such as 300M, welding is not recommended. Other common welding processes

0.002 0.004 0.006 0.008 0.010 Depth From Surface (in.)

Fig. 5.13. Effects of Grinding Burns on High Strength Steel12

0.002 0.004 0.006 0.008 0.010 Depth From Surface (in.)

Fig. 5.13. Effects of Grinding Burns on High Strength Steel12

for high strength steels include friction welding and pressure welding. Post-weld tempering is necessary for many processes to prevent cracking.

Many high strength steels are susceptible to stress corrosion cracking when placed in-service. The plane strain stress corrosion cracking fracture toughness of many high strength steels is only about one half that of the non-exposed fracture toughness (KIcscc ~ 1/2 KIc). Surface coatings, such as cadmium or chromium plating, are normally used to prevent access to the environment and

4340 Steel 52-53 Rc

Rotating Beam Fatigue

4340 Steel 52-53 Rc

Rotating Beam Fatigue

Fig. 5.14. Fatigue Improvement Due to Shot Peening

sacrificially corrode instead of the base metal. Hard chrome plating is often used where wear surfaces are involved. However, both of these plating methods can cause hydrogen embrittlement so it is important to stress relieve before plating and then bake immediately after plating to remove any hydrogen. If the steel is subject to possible hydrogen pickup due to pickling or electroplating operations and it has been heat treated to 200 ksi, or higher, it should be immediately baked at 365-385° F for at least 8 h or for 24 h if it is thicker than 1.5 in. Shot peening before chrome plating can significantly improve the fatigue life, as shown in Fig. 5.14. Shot peening induces a residual stress pattern (Fig. 5.15) near the surface that helps prevent plating cracks from propagating into the base metal.

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