Views: 132 Author: Site Editor Publish Time: 2018-06-14 Origin: www.fuchun-casting.com
Pouring process refers to the method and technology of pouring molten steel into steel ingots.
Pouring process is a crucial link in the whole production process of steel castings.The pouring process is not simply pouring the metal liquid into the cavity, because the molten steel in the process of melting, casting, and solidification will be connected with the surrounding medium (such as air, various refractory materials, etc.). A series of physical and chemical reactions will occur due to the impact of high temperature and fluid. Impurities, they will remain in the various parts of the casting.
Therefore, in steel casting production, we should strictly follow the requirements to carry out various operations in the casting process.
First of all, adequate preparations should be made before pouring to ensure smooth operation of the casting.
Preparatory work before pouring includes:
a. Clean up the casting site;
b. Check the situation of the ladle and drying preheating, as well as the flexibility and reliability of the transport and tilting mechanism.
c. To understand the types of cast alloys, to estimate the quantity of the products to be poured and the weight of the liquid steel needed, to prevent the shortage of molten steel and the insufficient cast type in the casting.
In order to obtain qualified steel castings, it is very important to control the pouring temperature, pouring speed and strictly abide by the casting operation rules.
a. As far as pouring temperature is concerned, the pouring temperature has a great influence on the quality of the castings. The reasonable pouring temperature range should be determined according to the type of alloy, the structure of the casting and the characteristics of the mold. The reasonable pouring temperature is selected according to the type of carbon steel. The general pouring temperature is between 1540 ~1580℃.
b. In terms of pouring speed, under the condition of smooth gas discharge in the guaranteed cavity,for castings requiring simultaneous solidification, a higher casting speed may be adopted, and for castings requiring sequential solidification, a lower casting speed may be adopted as far as possible.
c. It is necessary to specify the pouring time to meet the pouring speed, that is to say, starting from the design of the cross section area of the gate.
d. The casting of organic binder sand mold is faster than inorganic binder.
e. Generally speaking, the following requirements should be observed for casting operation requirements:
1. Pouring large and medium steel castings, steel need to be placed 1min to 2min before pouring.
2. After casting solidification, iron and box cards should be removed in time to reduce shrinkage resistance and avoid casting.
In order to obtain qualified castings and prevent impurities from occurring, it is necessary to adopt corresponding pouring process according to the characteristics of the castings and metals. Therefore, the design of gating system plays a vital role in casting quality, so do not despise it.
This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.
For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.
There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.
The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.
Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:
In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.
Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.
Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.
Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.
Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.
In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.
Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite.
It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1.
It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.
In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.
Martensitic, ferritic, duplex and precipitation hardening steels are magnetic.