90 THE DISTRIBUTOR’S LINK ROB LaPOINTE FASTENER SCIENCE: THE ROLE OF CARBON IN STEEL from page 14 Low Alloy Steel Low alloy steel consists of low carbon, medium carbon, and high carbon steels where carbon is the main alloying element. Low carbon steel (0.04-0.3 % C) has the lowest hardenability of low alloy steels. It is characterized by its flexibility, machinability, and low cost. SAE J429, grades 1 and 2 can be produced from low carbon steel and do not need heat treatment to achieve the tensile performance characteristics of 60,000 psi and 74,000 psi respectively. FIGURE 3 SAE J429, GRADE 8 (MEDIUM CARBON) HEX CAP SCREW. FIGURE 2 SAE J429, GRADE 2 (LOW CARBON) BOLTS. NO HEAD MARKING REQUIRED. Medium carbon steel (0.3-0.7 % C) is characterized by a higher cost than low carbon steel and has good hardenability through heat treatment involving a quench and temper process. Medium carbon steels are often used to produce SAE J429, grade 5 and 8 fasteners as well as ISO 898-1, classes 8.8 and 10.9 bolts, screws, and studs. High carbon steel (0.7-1.7% C) has greater hardenability than medium carbon steel for applications requiring greater wear resistance and strength. This added hardenability comes at the expense of flexibility and cost. Applications suitable for high carbon steel include cutting tools, knives, springs, and railway tracks. High Alloy Steel High-alloy steels have additional alloying elements beyond carbon to improve the alloy’s workability, hardenability ware resistance, and corrosion resistance. Alloying elements include silicon (Si), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), Nickel (Ni), copper (Cu), and molybdenum (Mo) among others. Often simply called alloy steel, high alloy steels can be used in producing SAE J429, grade 8, ASTM A574 socket head cap screws, and ISO 898-1, classes 8.8, 10.9 and 12.9 fasteners. Stainless fasteners are produced from high alloy steel with high chromium and nickel content. Specifically, it is the high chromium content that gives stainless its corrosion resistance. During the process of passivation, iron is removed from the surface of the alloy by interaction with an acid and chromium is left to form a protecting layer of chromic oxide (Cr2O3) which shields the material from oxidation of iron (red rust) deeper under the surface. See Figure 4. FIGURE 4 PASSIVATION PROCESS FOR STAINLESS STEEL, WHERE CHROMIC OXIDE FORMS ON SURFACE AND PREVENTS IRON OXIDES FROM FORMING BELOW. CONTINUED ON PAGE 132
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