【正文】 pe,and wire. A large portion of the material is cold worked in its final processing to improve its hardness, strength, and surfacefinish grades containing 20 points or less of carbon are susceptible to considerable plastic flow and are frequently used as deepdrawn products or may be used as a ductile core for casehardened material. The low lain carbon steels are reality brazed, welded, and forged.Medium Carbon. The medium carbon steels (%～%)contain sufficient carbon that they may be heat treated for desirable strength, hardness, machinability, or other properties. The hardness of plain carbon steels in this range cannot be increased sufficiently for the material to serve satisfactorily as cutting tools,but the loadcarrying capacity of the steels can be raised considerably, while still retaining sufficient ductility for good toughness. The majority of the steel is furnished in the hotrolled condition and is often machined for final finishing. It can be welded,but is more difficult to join by this method than the low carbon steel because of structural changes caused by welding heat in localized areas. High Carbon. High carbon steel contains from 50 to 160 points of carbon (%～%). This group of steels is classed as tool and die steel, in which hardness is the principal property desired. Because of the fast reaction time and resulting low hardenability, and its associated danger of distortion or cracking, it is seldom possible to develop fully of heattreathardened plain carbon steel is low pared to that of alloy steels with the same strength, but, even so, carbon steel is frequently used because of its lower cost.ALLOY STEELSAlthough plain carbon steels work well for many uses and are the cheapest steels and therefore the most used, they cannot pletely fulfill the requirements for some work. Individual or groups of properties can be improved by addition of various elements in the form of alloys. Even plain carbon steels are alloys of at least iron, carbon, and manganese, but the term alloy steel refers to steels containing elements other than these in controlled quantities greater than impurity concentration or, in the case of manganese, greater than %.Alloys Affect Hardenability. Interest in hardenability is indirect. Hardenability is usually thought of most in connection with depthhardening ability in a full hardening operation. However, with the isothermal transformation curves shifted to the right, the properties forging operations, the materially usually air cools. Any alloy generally shifts the transformation curves to the right, which with air cooling results in finer pearlite than would be formed in a plain carbon steel. This finer pearlite has higher hardness and strength, which has an effect on machinability and may lower ductility.Weldability. The generally bad influence of alloys on weldability is a further reflection of the influence on hardenability. With alloys present is a further reflection of the influence on hardenability. With alloys present during the rapid cooling taking place in the welding area, hard, nonductile structures are formed in the steel and frequently lead to cracking and distortion.Grain Size and Toughness. Nickel in particular has a very beneficial effect by retarding grain growth in the austenite range. As with hardenability, it is the secondary effects of grain refinement that are noted in properties. A finer grain structure may actually have less hardenability, but it has its most pronounced effect on toughness。 for two steels with equivalent in the chart as improved toughness. This improved toughness, however, may be detrimental to machinability. Corrosion Resistance. Most pure metals have relatively good corrosion resistance, which is generally lowered by impurities or small amounts of intentional alloys. In steel, carbon in particular lowers the corrosion resistance very seriously. In small percentages, copper and phosphorus are beneficial in reducing corrosion. Nickel bees effective in percentages of about %, and chromium is extremely effective in percentages greater than %,which leads to a separate class of alloy steels called stainless steels. Many tool steels,while not designed for the purpose, are in effect stainless steels because of the high percentage of chromium present.LOW ALLOY STRUCTURAL STEELS Certain low alloy steels sold under various trade names have been developed to provide a low cost structural material with higher yield strengh than plain carbon steel. The addition of small amount of some alloying elements can raise the yield strength of hotrolled sections without heat treatment to 30%～40% greater than that of plain carbon steels. Designing to higher working stresses may reduce the required section size by 25%～30% at an increased cost of 15%～50%,depending upon the amount and the kind of alloy.The low alloy structural steels are sold almost entirely in the form of hot rolled structural shapes. These materials have good weldability, ductility, better impact strength than that of plain carbon steel, and good corrosion resistance, particularly to atmospheric exposure. Many building codes are based on the more conservative use of plain carbon steels, and the use of alloy structural steel often has no economic advantage in these cases.LOW ALLOY AISI STEELSImproved Properties at Higher Cost. The low alloy American iron and steel institute (AISI) steels are alloyed primarily for improved hardenability. They are more costly than plain carbon steels, and their use can generally be justified only when needed in the heattreathardened and tempered condition. Compared to plain carbon steels, they can have 30%～40% higher yield strength and 10%～20% higher tensile strength. At equivalent tensile strengths and hardnesses, they can have 30%～40% higher reduction of area and approxi