Quality difference between forgings and castings
作者：admin 来源：本站 发布时间：2019-03-29 02:31:21 浏览量：
In the casting process, the metal casting into the mould cavity may produce shrinkage, separation or pore during solidification, which makes the casting unacceptable in the harsh environment application field.
Shrinkage occurs in two processes, when the metal is cooled at a temperature higher than the melting point, it shrinks further during solidification. For the first time, the compensation of molten metal is increased, but the compensation of solid cooling depends on the increase of size. Separation or chemical separation of melts occurs during solidification after solidification of a layer on the inner wall of the die cavity. During a long period of temperature change, low fluidity causes small solid particles - crystals - to form and grow in a dendritic structure. The initial crystals are close to the inner wall of the die cavity, with the lowest alloy content. In the core part, the alloy content is relatively high, which makes the expected composition have little similarity. Microsegregation also exists in each branch. As a result, micropore, precipitation of regenerated phase and mixing of metal and non-metal components were induced. During the cooling process, the gas escaping from the solution results in porosities, or is trapped between crystal branches to form tiny pores. In addition, as solidification and volume shrinkage of crystals, alternatives to melts will surely flow through a tortuous path along the staggered crystal network. Flow resistance may be too high, leading to microporous and porous. Other shortcomings in the casting are clear cracks and thermal tearing during solidification under the combined action of stress concentration caused by inhomogeneous shrinkage and low strength of metal near melting point temperature. Cold scars are formed at lower casting temperatures, and the accumulation of sand or slag in molten metals can lead to stains. Low-grade casting operations may also cause other defects.
Compared with castings, forgings have more uniform structure, better density, better strength integrity, better size characteristics and smaller size error. Directional structure has higher strength and stress performance than castings. High strength hot forging promotes crystallization and grain refinement, enabling materials to achieve maximum possible strength and consistency, and minimal variation between parts. Structural integrity forging eliminates internal defects, produces coherent metallographic structure and ensures excellent performance. Where there are serious stress and corrosion problems in crystals, forgings can ensure long service life and trouble-free service. The creep fatigue strength of forgings is more than three times higher than that of castings under the condition of great temperature change.
Generally speaking, forgings are superior to castings in all aspects, but relative cost, special-shaped products, processing difficulty and other aspects are still different, each has its advantages and disadvantages. In terms of density alone, the density of forgings is greater than that of castings.
