Truss and Analysis

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Metal Heat Treat – 2

Hardening versus Tempering

 

Hardening is achieved by heating a metal part to a predetermined temperature and then removing the heat rapidly via quenching. The quenching media can be water, brine or oil. Air cooling can produce hardening for some steel as well. Hardening is reserved for applications requiring a tough surface to withstand impact, superior fatigue life and increased strength. During hardening the metal looses some of its ductility and becomes brittle. As the hardness of a metal is increased, the brittleness increases as well.

 

Tempering is a process which removes some of the hardness off the metal but it restores some of its ductility. This is accomplished by heating the piece, in the case of steel from 400F up to 639F, depending on the desired properties. The soaking periods for either hardening or for tempering various metals are listed in available literature, as well as the rate of cooling to achieve required hardness.

 

Note that introduction of alloys into steel, reduces the cooling rate required to harden the steel alloy. Therefore, the brittleness is reduced while the toughness is achieved, without warping and cracking taking place. This process is called “heat treatment” rather than “hardening”.

When comparing hardness achieved by carbon steel versus alloy steel of the same content, the alloy steel is more effective in achieving hardness, and strength. Alloy steels are superior to plain carbon steels.

 

Not all steels can be hardened. Low carbon steels, pure iron and wrought iron can not be hardened. Cast iron is not a candidate for hardening. When it is cooled rapidly, it forms white iron, which is hard and brittle under impact. When it is cooled slowly, it forms gray iron which is soft and brittle under impact.

 

Increasing the carbon content of steel increases the maximum hardness which can be achieved on a steel piece, up to a certain point. The range of carbon required for hardness reaches up to .8%. Beyond that level of carbon concentration, there is no benefit to hardening of the steel piece, though the wear resistance is increased.

Peter K. Theodorakakos

 

Metal Heat Treat – 1

Normalizing versus Annealing

 

Both, Normalizing and Annealing of a metal part, refer to a Heat Treating process which can alter its physical and chemical properties by increasing ductility and reducing hardness to improve machinability.  Atoms migrate and dislocations are reduced creating a uniform grain, and thus resulting in material property alteration.

 

The Normalizing process features the lower temperature of the two. Normalizing is applicable to Ferrous Metals only, and it is best suited for high carbon content steels which require lower temperatures for relaxation. Subjecting a part to such heat treatment relaxes the metal from residual stresses which are formed during machining, casting, welding, or forging of the part. Removal of residual stresses improves fatigue life of the part by relieving high stress areas which can form cracks, and foster their propagation. Normalizing must be considered before a part is processed for hardening. Normalizing low carbon steel is of no benefit, nor is it harmful. Normalizing is considered for parts which will be subjected to impact or need to have a tough surface. Normalizing involves heating a part to a predetermined temperature and allowing it to air cool. A thick part cools much slower than a thin one due to the heat capacitance of the metal. So, a thin part is potentially being hardened as it cools. An advantage to normalizing steel is the robust dimensional stability it exhibits if further heat treatment takes place, as well as, improved machinability.

 

The Annealing process involves heating a part to a predetermined temperature and slowly cooling it inside an oven at a controlled environment. Therefore, annealing is preferred if predetermined hardness is a significant property of a metal piece.

Annealing is applicable to Nonferrous Metals, such as Copper, Brass, Silver and Aluminum which become brittle when they are mechanically worked. After annealing, they become soft again. Several alloys of aluminum exist and each requires a special heat treatment to augment its properties. Annealing affects both copper and aluminum in a similar manner. Though it relaxes them as intended, the drawback is “hot shortness”. Their Tensile Strength is diminished at temperatures over 900F and the subject parts can break unless a support is provided in the form of a fixture.

Annealing is reserved for low carbon content steel, which requires high temperatures for relaxation. As the carbon content of the subject steel is increasing, the corresponding annealing temperatures drop.

When annealing steel, after it is heat soaked it is slowly cooled, whereas for Non Ferrous metals cooling rate can be slow or accelerated by quenching in water or oil.

Peter K. Theodorakakos PE, CQE, 6 Sigma BB