Transcript Slide 1
Ultra high strength steels RG 1 Introduction in ULTRA HIGH STRENGTH STEELS Conventional direct hardening steels are usually designed ranges of tensile strength, which in commercial practice ranges from 75 or 100 MPa. For example, 8501100 MPa. General effect of Tempering Temperature on the Room Temperature Mechanical Properties of Steels Initially Quenched to be Almost Completely Martensitic. In commercial engineering applications, the tensile strength is usually limited to a maximum of 1250 MPa, with a few exceptions to 1500MPa. Above 1500 MPa, steels are considered to be ultra-high tensile with 2200MPa being a limit for conventional quench/temper heat treatment RG 2 practice. RG 3 Remelted Ultra-High Strength Steels Electro-Slag Remelting (ESR) and Vacuum Arc Remelting (VAR) are processes often used to improve the purity and reduce the inclusion content of ultra-high strength alloy steels. They involve total remelting of a consumable “Electrode” in the form of a specially prepared ingot of the steel quality to be remelted. ESR is a reactive process whereby sulfur is reduced and was used for that purpose before ladle technology was developed to its current level. Inclusions are removed by chemical and physical means. Even though the inclusion content is less, the oxygen content is higher than air melted steels. Nitrogen levels do not change significantly. Hydrogen can increase if an inert atmosphere is not used but steelmakers know how to remove it later by heat treating the product. RG 4 VAR removes oxygen, while hydrogen and nitrogen can remove sulfur, although the feedstock ingots for the process are often made to very low sulfur levels, 0.005%S max. The process are slow (approximately – 1 tonne/h) hence relatively high costs are incurred (typically 3 times more expensive than the conventional condition). RG 5 • • • Solution hardening. Except for nitrogen, which dissolves as an interstitial like carbon, all other suitable elements will always be of the substitutional solid solution type. Precipitation hardening. Either by forming finely dispersed hard and small carbides of the alloying elements, or by influencing the cementite formation to occur in fine particles, or by producing precipitates of compounds of the alloying elements (e.g. borides, or intermetallic phases), or by all of the above. Grain size reduction. You may produce small grains (i.e. from a martensitic transformation which rips on i "apart" in many grains), and/or keep small grains small by keepig grain boundaries from moving (i.e. grains from growing) by precipitating suitable elements there (without making the grain boundary brittle, of course). This will always lead to hardening, too. RG Hardening mechanisms of maraging steel Solution hardening in alloy steels 6 RG 7 Processing of Ultra-High Strength Steels Processing of ultra-high strength involves tighter disciplines than are normally adopted for lower strength direct hardening alloy steels. Processing stages for ultra-high strength steels RG 8 Maraging steels Maraging steels are special ultra-high strength steels that achieve high levels of strength and toughness quite differently from conventional alloy steels. Whilst the structure is low carbon (lath) martensite, their high hardness and strength are produced by the precipitation of intermetallic compounds throughout the matrix. Tensile strength Fracture Toughness of Maraging Steels and Other High-Strength Martensitic Grades. RG 9 Tipical chemical composition and mechanical properties of maraging steels Maraging steels have very low carbon contents (0.01-0.03 max.) and restricted silicon and manganese levels. The major alloying elements are nickel, cobalt, molybdenum and titanium. Like other ultra-high strength steels, they are invariably vacuum arc remelted. RG 10 This iron-based family of steels are based on a composition of iron and 18 % nickel with additions of cobalt, molybdenum and titanium plus other elements. The most frequently specified 'C' grades contain a significant amount of cobalt. There are also 'T' (titanium) grades which contain no cobalt and have a lower molybdenum content and a greater addition of titanium compared with the corresponding strength 'C' grades. RG 11 The structure of the alloys are formed of fine martensite which then undergoes an aging process (precipitation hardening) giving them the name 'mar-aging' steels. Maraging steels are characterised by ultra-high strength, simple aging treatment which minimises distortion, high levels of toughness, moderate corrosion resistance (similar to that of standard martensitic steels), good machinability (usually in the annealed condition) and good weldability. RG 12 RG 13 APPLICATIONS OF MARAGING STEELS Applications of are always where the unique strength/weight ratio is vital; this is done in order to justify the very high cost of these steels. They are used in ultra-high strength aircraft components and find a strategic military role in lightweight bridges, etc. Formula One Racing structural components are often made from these steels. In addition to superb mechanical proporties there is little distortion experienced during the aging process. Maraging steels are diverse and include missile casings, tooling, ordnance mounting blocks, high performance autosport components, couplings, bearings, load cells, landing gear components, transmission shafts, jet engine components and helicopter drive shafts. RG 14 RG 15 RG 16