STAINLESS STEEL FORGINGS Contents Introduction ............................................................ 5 Forging Terminology .............................................. 9 Design Considerations ......................................... 13 Tolerances ........................................................... 17 Quality Descriptions and Special Requirements .......................................... 21 Nondestructive Product Inspections........................................................... 23 Typical Properties of Wrought Stainless Steel ........................................24 1 FORGING RANGES FOR STAINLESS STEELS Temperature, °F 1400 1600 1800 2000 2200 2400 Type 440C Type 347 & 348 Type 321 Type 440B Type 440A Type 310 Type 310S Type 329 Type 317 More Difficult to Hot Work More Difficult to Hot Work Type 316L Type 316 Type 309S Type 309 Type 303 Type 303 Se Type 305 Type 302 & 304 Type 431 Type 414 Type 420F Type 420 Type 416 Type 410 Type 446 Type 443 Easier to Hot Work Type 430F ⇐ Type 430 Note: This chart does not take into consideration aspects of hot working such as heating and cooling practices, scaling rate, grain size, billet size and equipment. It should not be used as a basis for selecting materials without metallurgical advice. F − 32 C = 1.8 Source: METAL PROGRESS, June 1974 2 Preface Designers select stainless steels first on the basis of corrosion resistance, then on the basis of strength and other mechanical properties. In the interest of achieving optimum quality at the most economical cost, designers do not overlook a third factor, manufacturing. Fabrication is important even in early stages of design, and forging is one method of fabrication that designers regularly consider. The reason for this is that stainless steels have advantages that are difficult to duplicate, and forging enhances these advantages, which include: Corrosion and Heat Resistance The principal advantage of stainless steels is resistance to corrosion by moisture, atmospheric conditions, many acids, and other aggressive environments at low or high temperature. Strength Parts made of stainless steel are often stronger and tougher than parts made of mild steels or nonferrous metals. Grain Structure A unique feature of forgings is the continuous grain flow that follows the contour of the part, as illustrated by the top drawing. In comparison is the random grain structure of a cast part (center) and the straight-line orientation of grain in a machined part (bottom). From this simple fact. stem many secondary advantages inherent in forged stainless steels: Strength where needed. Through grain refinement and flow, forging puts the strength where it's needed most. Lighter weight. Higher strength-to-weight ratio permits the use of thinner, lighter weight sections – without sacrificing safety. Improved mechanical properties. Forging develops the full impact resistance, fatigue resistance, ductility, creep-rupture life, and other mechanical properties of stainless steels. Repeatable dimensions. Tolerances of a few thousandths are routinely maintained from part to part, simplifying final fixturing and machining requirements. Efficient metal utilization. Forging cuts waste because it reduces metal removal. Structural uniformity. Forgings are sound, nonporous, and uniform in metallurgical structure. Availability Wide choice of stainless steel types. With few exceptions all stainless steels can be forged, as suggested by the chart (opposite page) and by the many applications illustrated in this booklet. Wide range of sizes and shapes. Forgers make stainless steel parts from a few ounces in weight to hundreds of pounds; smaller than one inch to parts many feet long. Special operations such as extrusion, drawing, piercing, and coining further enhance forging capabilities. 3 The three components shown here are for aircraft applications, illustrating one often overlooked aspect of stainless steels: Because of their high strength-to-weight ratios, stainless steels serve for light- weight design applications just as well as other light-weight materials. For example, the long part, above, is a structural compo- nent for the cargo version of the Boeing 747 Jumbo Jet. It is forged of Type S15500 precipitation hardening stainless steel, measures about 23" long, and weighs 8 pounds. Below that is a 4.2-pound bracket forged of Type 410 stainless steel. It is a motor mount bracket for the 250-Series aircraft engines produced by Detroit Diesel, Allison Division of General Motors Corporation. The bottom photograph is a 7-pound component for the F-4 Phantom Jet pro- duced by McDonnell Douglas Corporation. It is forged of Type S13800 precipitation -hardening stainless steel. Forging achieves the best in strength-to-weight ratios in stainless steel parts. Courtesy Consolidated Industries, Inc., Cheshire, Connecticut This mechanical linkage part illustrates the extent to which forging reduces machining. Not only is it a difficult shape to machine, but machining would result in considerable (about 40%) metal scrap. The part is Type S17400 precipitation hardening stainless steel that combines high strength and hardness with excellent corrosion re- sistance. Courtesy Cornell Forge Company, Chicago, Illinois This bearing housing for a rocket aircraft was forged to minimize machining and to provide optimum mechanical properties. Stain- less steel was selected for its resistance to corrosion. The stainless steel is Type 410, and the part was impression die forged on a 2500- pound hammer. Courtesy Cornell Forge Company, Chicago, Illinois The forging bar for this helicopter sling hook is first bent then impres- sion die forged on a 2,000-pound hammer. Grain flow is in the shape of the hook for maximum strength, which is essential for a part like this subjected to high stresses during maximum loading. The stain- less steel is Type S17400. Courtesy Endicott Forging & Mfg. Co., Inc., Endicott, New York 4 By way of introduction . What is stainless steel? Stainless steel is not just one material but a family of many different, but related corrosion resistant steel alloys containing about 10.5% chromium and up. Other alloying elements beside chromium may be present in stainless steel. These include nickel, manganese, molybdenum, and others. American Iron and Steel Institute (AISI) designates 57 stainless steels as standard compositions. All are listed in Table 1 on page 24. A more detailed description of each type is contained in the AISI publication, “Steel Products Manual–Stainless and Heat Resisting Steels.”1 Also, many special analysis stainless steels are produced in the United States that do not have AISI designation numbers. Many of these are identified in technical literature, such as in the ASTM Data Series Booklet DS 45.2 Corrosion resistance is the outstanding characteristic of stainless steels and the principal reason for their use. These steels are not immune to attack in all environments; however, their performance is outstanding when compared with ordinary steel and other common metals. Table 2 on page 32 gives some indication of the relative corrosion resistance of stainless steels to seven typical environments. How are stainless steels identified? Those not familiar with stainless steels often ask this question, because there are different terms used that tend to cause confusion. For example, the terms austenitic, martensitic, ferritic, and precipitation hardening serve to identify categories of stainless steels on the basis of their metallurgical structure. Design and product engineers should recognize these terms and understand what they mean, because the stainless steels so classified tend to have similar characteristics with respect to corrosion resistance, hardenability, and fabricability. AISI stainless steels are identified by a system of numbers that are in either 200, 300, or 400 Series. The 200 Series stainless steels contain chromium, nickel, and manganese; the 300 Series contain chromium and nickel; while the 400 Series are straight-chromium stainless steels. This numbering system is the one by which most people today identify stainless steels, such as Type 304 or Type 316, etc. A new Unified Numbering System (UNS) has been developed that applies to all commercial metals, including steels, nonferrous metals, and even to rare earths. The Society of Automotive Engineers (SAE)3 and the American Society for Testing and Materials (ASTM)4 developed the system, and AISI is cooperating in the effort to have UNS apply to all steels. Accordingly, UNS numbers appear with the AISI type numbers in Table 1. Note in this table that five of the stainless steels are identified by UNS numbers only. 5 The operational environment inside a jet engine consists of three basic ingredients . heat, pressure, and airflow. All three can only be described as severe. No wonder, then, that so many inside components for jet engines are forgings, such as the fuel-nozzle support shown here. This 4" high, 1½ pound forging is mounted in the combustion chamber, or burner, of huge turbofan jet engines used to power one of the popular wide-body airliners. Jet fuel flows through the support to a sophisticated nozzle, which sprays the fuel into the burner, where it is ignited and converted into thrust energy. Made of Type 347 stainless steel, this support takes shape in four forging operations: upset, bent, blocked, and then finish-forged and trimmed in a closed-impression die. As-forged weight is 2½ pounds. After being machined at the nozzle tip and mounting flange, the