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What Is Heat Treatment?What Happens When Metals After Heat Treating?
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What Is Heat Treatment?What Happens When Metals After Heat Treating?

Views: 51     Author: Site Editor     Publish Time: 2018-05-03      Origin: www.fuchun-casting.com

What Is Heat Treatment?

Heat treatment is the heating and cooling of metals to change their physical and mechanical properties, without letting it change its Heat Treatment shape. Heat treatment could be said to be a method for strengthening materials but could also be used to alter some mechanical properties such as improving formability, machining, etc. The most common application is metallurgical but heat treatment can also be used in manufacture of glass, aluminum, steel and many more materials.

 

Heat treatment processes include case hardening, tempering, solution and ageing treatment, Specialty Stainless Steel Processes (S3P), annealing and normalising.

 

Heat treated parts are essential to the operation of automobiles, aircraft, spacecraft, computers and heavy equipment of every kind. Saws, axes, cutting tools, bearings, gears, axles, fasteners, camshafts and crankshafts all depend on heat treating.

 

The Most Common Heat Treatment

Case HardeningCase hardening is the process of hardening the surface of a metal by infusing elements into the material's surface, forming a thin layer of a harder alloy.

 

AnnealingAnnealing is a form of heat treatment that brings a metal closer to its equilibrium state. Annealing softens metal making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.

 

NormalisingNormalizing involves heating steel, and then keeping it at that temperature for a period of time, and then cooling it in air. The resulting microstructure is a mixture of ferrite and cementite which has a higher strength and hardness, but lower ductility. Normalizing is performed on structures and structural components that will be subjected to machining, because it improves the machinability of carbon steels.

 

The main difference between normalizing and annealing is that the cooling rate of normalizing is slightly faster,so the production cycle of normal heat treatment is short. Therefore, normalizing can be used as far as possible when annealing and normalizing can meet the performance requirements of parts.


 

The Basics of Heat Treating
Although iron and steel account for the vast majority of heat treated materials, alloys of aluminum, copper, magnesium, nickel and titanium may also be heat treated.

Heat treating processes require three basic steps:

1. Heating to a specified temperature.

2. Holding at that temperature for the appropriate amount of time.

3. Cooling according to prescribed methods.

Temperatures may range as high as 2400°F and time at temperature may vary from a few seconds to as many as 60 hours or more.

Some materials are cooled slowly in the furnace, but others must be cooled quickly, or quenched. Certain cryogenic processes require treatment at -120°F or lower. Quenching media include water, brine, oils, polymer solutions, molten salts, molten metals and gases. Each has specific characteristics that make it ideal for certain applications. However, 90 percent of parts are quenched in water, oil, gases or polymers.

heat treatment workshop


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FAQS

  • What is 'multiple certification'?

    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.

  • What surface finishes are available on stainless steels?

    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.

  • Can I use stainless steel at high temperatures?

    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:

    1. Maximum temperature of operation
    2. Time at temperature, cyclic nature of process
    3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
    4. Strength requirement

    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.

  • Can I use stainless steel at low temperatures?

    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. 

  • Is stainless steel non-magnetic?

    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.

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