英语翻译Some product applications require properties of hardness

英语翻译
Some product applications require properties of hardness and strength to resist penetration under high pressure and to provide maximum wear qualities only on the surface .Where through hardness and the maxium strength associateed with it are not necessary,it may be more economical to gain the needed surface qualities by a casehardening process.Casehardening involves a change of surface properties to produce ahard,wear-resistant shell about atough,fracture-resistant core.
Casehardening of steel may be accomplished by a number of methods.Choice between them is dependent on the material to be treated,the application,and the desired properties.One of the more common methods is carburizing,which implies an increase or addition of carbon which is actually the basis of the prcess.
Performed on Low Carbon Steels.Carburizing is usually performed on a low alloy or plain low carbon steel.If an alloy stell is used,it usually contains small quantities of nickel or some other element that acts as a retardant to grain growth during the heating cycle.Low carbon steels are commonly used to minimize the effect of subsequent heart tretments on the core material.It is possible to carburize any steel containing less than the 0.70%~1.20% carbon that is produced in the surface material.
Carbon Diffusion Is Time-temperratrue Dependent.Carbon is caused to diffuse into the steel by heating the material above its critical temperature and holding it in the presence of excess carbon.Temperatures used are usually between 850℃and930℃,with the chocice most dependent on the desired rate of penetration,the desired sutface carbon content,and the permissible grain growth in the material.Pentration is dependent upon both the temperature and time,with variation of case depth from 0.25 to 1.0 millimeter(0.010-0.040 inch) possible in the first 2 hours by varying the tempereature between the two extremes.The rate of penetration slows down as the depth increases,as shown in Fig.11-1,so that for large depths,relatively long periods of time are necessary.