Many Grades. Many Applications.
There is a classification of steels, alloy steels, AISI/SAE, that covers some one hundred grades, from a nominal 0.10% carbon, through to a nominal 1.0% carbon. The grades are used in myriad applications in the aerospace, agricultural, automotive, defense, and oil and gas industries. They are also used to make bolts, studs and nuts in many sizes and strengths, together with bearings.
AISI 4130 and AISI 4140 grades are workhorses among alloy steel forgings. They have many uses in the automotive, agricultural, and defense sectors, as forged gears, shafts, spindles, fixtures, jigs and collars. AISI 4130 is used in the oil and gas industry as forged valve bodies and pumps. It will be noted at the outset that the majority of these alloy steel grades contain a small amount of molybdenum, an addition that will increase strength, but will also prevent the occurrence of temper brittleness. This is a condition noted in steels that are slow cooled from 1100ºF (590ºC) or tempered in the range between approximately 850 and 1100ºF (450 and 590ºC). Molybdenum has a definite retarding effect on this phenomenon. In any event, tempering below 850ºF or above 1100ºF followed by a water quench is recommended.
AISI 4330V, for example, a modified 4330 grade, is a nickel-chromium-molybdenum steel with a small vanadium addition. The addition, 0.05/0.10% vanadium, results in a fine grain size, and increased tensile strength and impact resistance, at both ambient and at sub-zero temperatures. The steel is used in the oil and gas industry, typically for oil tools and drilling jars. Its high strength and impact resistance allow its use as landing gear components. This is just one example of the uses of alloy steel forgings.
There are many grades with carbon contents in the range 0.10 to 0.20%. These are primarily case-hardening grades (carburizing, nitriding, induction hardening) that come off the forging operation effectively ready to be machined. They need no annealing or normalizing prior to surface hardening and subsequent hardening and tempering, unless there are significant structural or section changes in the forged part. In such a case, the steel will definitely benefit from a normalizing treatment. In fact, machinability in some grades is better in the as-forged condition. Good machinability of these grades coincides with a microstructure of coarse pearlite to coarse spheroidite. The case-hardening treatment on these grades results in high surface hardness and high core toughness. The grades are typified by AISI 8615 and 8620, nickel-chromium-molybdenum steels, that may be used for the manufacture of forged camshafts, fasteners, and gears.
Forged parts with a nominal carbon over 0.25% are annealed by a slow cool from the forge or annealed in a furnace held at the approximate forging finishing temperature, where they are soaked and air cooled.
AISI 52100 is a high-carbon, chromium-bearing steel that has excellent strength and fatigue properties. It is used for aircraft bearings and for other highly stressed parts. For the more demanding applications, vacuum-arc-remelting is preferred, to give optimum performance. It has the good rolling fatigue strength required at operating temperatures less than 400ºF (200ºC).
AISI 9260 is a high silicon steel, about 2%, that is used for its spring properties in the automotive and general engineering sectors. AISI 94B30 and AISI 94B40 have a boron content of 0.0005% to 0.003% for increased hardenability. Both grades have relatively low nickel, chromium, and molybdenum contents. The boron takes care of the hardenability.
The whole range of alloy steels is dependent upon calculated carbon, nickel, chromium, and molybdenum content to allow for required mechanical property values, hence their suitability for their respective end uses. But chemistry alone will not guarantee this suitability. It takes correct heating to forging temperature, forge reduction techniques, cooling from forging, and subsequent heat treatment.
From annealing to normalizing, to surface hardening, and to final hardening and tempering, heat treatment practices are continually being monitored, and where necessary, revised. There will be changes to times and temperatures, and possibly installation of new heat treat furnaces. The one constant in this whole equation is the personnel running the furnaces, and the years of experience tucked under their belts. To do the best job possible, the heat treaters will understand the importance of the forging operation they’re following on, and the end use of the steel they’re treating. Here again, end use knowledge at the outset has a direct impact on the successful fabrication of the forging and subsequent heat treating.
AMFG personnel are conversant with the melting and forging techniques necessary to produce alloy steels suitable for application. And with the information necessary to ensure a final heat-treated product.
