Views: 34 Author: Site Editor Publish Time: 2018-10-25 Origin: www.fuchun-casting.com
Around the 21st century BC, China entered the Bronze Age, which saw the growth and maturity of a civilization that would be sustained in its essential aspects for another 2,000 years.
Yin-Shang bronzes mark a peak in the late Bronze Age all over the world, with a variety of fine bronze wares, e.g., the 832.84 kilogram giant Houmuwu Ding bronze vessel of 133 centimeter height, 110 centimeter length, and 79 centimeter width.They were cast in a great variety of sophisticated shapes, mythical animals, natural objects and beautiful designs. Bronze wares have been found in the middle and lower reaches of the Yellow River and Yangtze Rivers and some remote border areas. Many of the bronze wares have names and/or stories written on them in ancient Chinese characters, providing key evidences for the well-developed Yin-Shang bronze civilization and history of China.
Seeing here,we all have a question, in that era of low productivity, how did the ancients make bronzes, and how did they achieve exquisite shapes?
The earliest Chinese bronzes were made by the method known as piece-mold casting—as opposed to the lost-wax method, which was used in all other Bronze Age cultures. In piece-mold casting, a model is made of the object to be cast, and a clay mold taken of the model. The mold is then cut in sections to release the model, and the sections are reassembled after firing to form the mold for casting. If the object to be cast is a vessel, a core has to be placed inside the mold to provide the vessel’s cavity. The piece-mold method was most likely the only one used in China until at least the end of the Shang dynasty. An advantage of this rather cumbersome way of casting bronze was that the decorative patterns could be carved or stamped directly on the inner surface of the mold before it was fired. This technique enabled the bronzeworker to achieve a high degree of sharpness and definition in even the most intricate designs.
Bronze wares, as precious metal wares, were mainly used for ceremonial purposes by the upper class in ancient China. The variety and quantity of the bronzes that were used were a reflection of the owner's social status and power. Bronze technology is one of the most important achievements of Chinese civilization.
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.