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Views: 71 Author: Site Editor Publish Time: 2018-05-30 Origin: www.fuchun-casting.com
Cast irons are a family of iron-carbon alloys. Their high carbon content (usually 2–4%) gives cast iron its characteristic hardness. To facilitate a better understanding of these materials, they can be divided into five groups, based on composition and metallurgical structure: white cast iron, malleable cast iron, grey cast iron, ductile cast iron and alloy cast iron.
The carbon in white cast iron exists in the form of cementite and its section is gray. It is a good anti-wear material and works under abrasive wear conditions.
White cast iron includes ordinary white cast iron, low alloy white cast iron, medium alloy white cast iron, and high alloy white cast iron. China has a national standard (G.B8263-87).
White cast iron is hard and brittle, and it is not easy to cut. It is seldom used directly in casting parts.It is used in applications where abrasion resistance is important and ductility not required, such as liners for cement mixers, ball mills, certain types of drawing dies and extrusion nozzles.
Malleable cast iron is a kind of cast iron with high toughness which is treated by graphitization or oxidative decarburization annealing.As graphite is floc like, the fracture and tip effect of the matrix are reduced, the strength and toughness of malleable iron are much higher than that of gray iron.It should be pointed out that malleable iron can not be processed by forging, but it is only a name.
Malleable iron is divided into Ferritic Malleable Iron, pearlite malleable iron and white core malleable iron, in which pearlite malleable iron has relatively high performance, and can be used to make crankshaft, gear and other wear-resistant parts instead of some steel.
Ferritic Malleable iron is usually used for making pipe fittings, pipe fittings and low and medium pressure valves.
White heart malleable iron is mostly used for thin-walled shell parts with good weldability and toughness.
Grey cast iron is one of the most widely used casting alloys and typically contains between 2,5% and 4% carbon and between 1% and 3% silicon. It has excellent casting property, machinability, abrasion resistance and shock absorption. The compressive strength and hardness are close to that of carbon steel, but the tensile strength and plasticity are low and brittleness is large. Grey cast iron is simple in production process and low in price. It is widely used in mechanical engineering and is mainly used for wear resistant components under moderate load.
Ductile cast iron is the best mechanical property. As graphite is spherical, it greatly reduces the separation and tip effect on the matrix. The mechanical properties are much higher than that of gray iron, and the strength is close to the steel, and there are some advantages of gray cast iron.
Such as better damping, friction reducing, low notch sensitivity, excellent casting and excellent machinability. The disadvantages are large shrinkage, large white chowing and poor fluidity. The requirements for raw materials and smelting and casting process are higher than those of gray cast iron.
Typical applications are agricultural (tractor and implement parts); automotive and diesel (crankshafts, pistons and cylinder heads); electrical fittings, switch boxes, motor frames and circuit breaker parts; mining (hoist drums, drive pulleys, fly wheels and elevator buckets); steel mill (work rolls, furnace doors, table rolls and bearings); and tool and die (wrenches, levers, clamp frames, chuck bodies and dies for shaping steel, aluminium, brass, bronze and titanium).
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.