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How To Lift The Tensile Strength Of The Casting
Jul 04, 2017

How to lift the tensile strength of the Casting

In order to obtain the high strength of gray cast iron, the tensile strength of the casting is guaranteed above 300N/mm2, and there are two methods for improving the strength. Method one: change the composition of the furnace material, increase the amount of scrap steel, thus reduce the carbon equivalent and the incubation in the bag. Method two: add a small amount of alloy element and inoculation with the measure. Method one due to reduced the content of carbon, silicon, its low carbon equivalent, reduced the number of graphite, refine the graphite, increased ChuXi austenite dendrite, thus improve the strength of cast iron, but at the same time reduce carbon equivalent can lead to reduce the performance of casting, casting section sensitivity increases, the casting internal stress increases, the hardness increased, processing difficulties and other issues. Method 2 used in front of the furnace to join a small amount of alloy elements, can be appropriately to promote the formation of a stable and strong casting internal pearlite, and part of the granular pearlite, strengthening ferrite, form the right amount of carbides, at the same time should be to keep the original iron liquid of high carbon equivalent, reduce the chilling tendency, casting performance is good, not easy to produce shrinkage cavity, shrinkage phenomenon. Via comparison, and then according to the characteristics of the casting structure (complex structure, high strength thin wall casting), so we decided to adopt the second method, and after a lot of analysis and comparison decided to choose copper and chromium alloy two clock used at the same time and the combined method of inoculation trial production of low alloying high strength gray cast iron.

Of low alloying high strength gray cast iron in order to ensure that our factory successfully trial-produced, finds out some effect in production of low alloying high strength gray cast iron manufacture some of the major problems, cause and effect analysis (see figure 1). Such as: raw materials, operation, methods, etc., and implements corrective actions for these major problems, specific fulfil staff and inspectors, so that these problems in the process of field production in advance under control, to ensure successful new product production.

. Stomatal class

Because the casting has serious oxidation and inspiratory tendency, also directly with in the process of smelting furnace gas or contact outside the atmosphere, as a result, such as smelting process control slightly improper, casting is easy to absorb the gas and gas hole, the most common is the pinhole. Pinhole (gas porosity/pin - hole), usually refers to less than 1 mm of precipitation in casting porosity, assumes the circular, uneven distribution on casting the whole cross section, especially in the Casting and the cooling speed of heavy section smaller parts. According to the distribution and shape characteristics of the exhalation hole of the casting, the pinhole can be divided into three types, namely:

(1) point pinholes: the needle holes in the lower tissue are round, and the pinholes are clear and discrete, and they can count the number of pinholes in the area of each square centimetre, and the diameter of the needle is measured. This pinhole is easy to distinguish from shrinkage and shrinkage.

(2) reticular pinholes: in the low-power tissue, the pinholes are densely connected into a network, and there are a few large holes, which are inconvenient to check the number of pinholes in the area of the unit area, and it is difficult to measure the diameter of the pinholes.

(3) comprehensive stomata: it is the middle type of pinhole and reticulated pinholes, and from the low - fold tissue, large pinholes are more, but not round, but polygonal.

Casting production practice has proved that the casting due to inhale formation porosity is the main gas composition of the hydrogen, and its emergence has no certain rules to follow, is often a furnace time all or most of the Casting have pinhole phenomenon; The materials are no exception, and the casting of various components can easily produce pinholes.

2. Formation of pinholes

When the Casting are smelting and pouring, they absorb a lot of hydrogen, and the cooling is precipitated by the drop of solubility. Some information (2), casting more hydrogen, is dissolved in the solubility increases with the rise of temperature and liquid alloy, decrease with temperature drop, from a liquid into a solid when the hydrogen solubility in the casting by 19 times. (the relationship between solubility and temperature of hydrogen in pure aluminum is shown in figure 1). Therefore, during the solidification process of cooling, the hydrogen content exceeds its solubility and precipitates out in the form of bubbles. In the process of solidification, small, scattered pores are formed by the formation of hydrogen bubbles formed by the over-saturated hydrogen, which is the common form of gas pores. The supersaturation reached before the hydrogen bubble is formed is a function of the number of hydrogen foam nuclei, while oxides and other inclusions are at the core of the bubble