6 Reasons Steel is Vacuum Treated

May 7, 2015

 Vacuum treated (vacuum degassed) steel is used for  critical applications that require steel with an exceptionally high degree of structural uniformity, internal soundness, and other characteristics which may be impaired by the effects of uncontrolled amounts of dissolved gases. Vacuum degassing treatments, along with various deoxidation practices are specified to control the amounts of dissolved gases in the steel. This post describes 6 benefits of vacuum treatment of steel.

 Liquid steel after treatment in a Siemens RH degassing plant achieves improved properties.


Liquid steel after treatment in a Siemens RH degassing plant achieves improved properties.

Vacuum treatment of molten steel

  • Reduces Hydrogen content. This reduces the tendency of steel to “flake” or become “embrittled.”
  • Reduces Oxygen content. This makes it easier for the steel to conform to restrictive microcleanliness requirements.
  • Improves the recovery and uniformity of distribution of alloying elements and other additives.
  • Helps control the composition of the steel closer than without vacuum treatment.
  • Results in higher and more uniform transverse ductility, improved fatigue resistance, and improved high temperature performance.
  • Can be used to achieve exceptionally low carbon content that are otherwise unobtainable by conventional means.

What are some situations where vacuum treatment is employed?

  • Large forgings and large cross sections where hydrogen would otherwise remain and contribute to flaking and embrittlement.
  • Bearings where uniformity throughout the section is important for critical performance.
  • Inverted delta, human critical safety applications where steel toughness and performance place high demands on the steels properties in all directions.

The removal of Oxygen by degassing  is a challenge for the steelmaker, because this element is extremely reactive- it can exist in the steel in many forms, such as free oxygen, dissolved in the melt as a soluble nonmetallic oxide, can combine with carbon to form gaseous oxides, and it can exist as complex oxides in the accompanying slags and refractories in the process.

In post will describe some steel deoxidation  practices and the types of vacuum degassing that are used in the North American steel Industry

Siemens Vacuum Degasser Photo


5 Reasons To Anneal Steel

June 29, 2010

Here are 5 reasons to anneal steel. 
To alter the grain structure;
To develop formability;
To improve machinability;
To modify mechanical properties;
To relieve residual stresses.
 

 
The annealing process is a combination of a heating cycle, a holding period or “soak” at temperature, and a controlled cooling cycle. Atmospheric controls are generally used to protect the steel from oxidation. 

The temperatures used and the cooling rates are carefully selected to correspond with each steel grade’s chemical composition in order to produce the results desired. 

For bar steels used in our precision machining shops, there are three kinds of annealing that may be encountered:
Subcritical Anneal
Solution Anneal
Spheroidize Anneal
 

Subcritical Anneal 

 A subcritical anneal is the metallurgical name for what is termed a process anneal or stress relief anneal in North American commercial practice. It consists of heating the steel to a temperature close, but below, the steel’s lower critical temperature or Ac1. This simple anneal reduces stress and hardness in the material and makes modest changes in its microstructure. Steel mills often employ this to improve cold shearing or cold forming. This is sometimes used between cold forming operations to reduce hardness. 

Solution Anneal 

Lamellar Pearlite.

Solution annealing is referred to in commercial practice as ‘LP Anneal’ or Lamellar Pearlite Anneal. Lamellar pearlite is the microstructure that predominates when doing this kind of anneal. The cycle for this anneal involves heating the material above the critical range (Ac3) and holding the steel (soaking) at that temperature for a length of time followed by slow cooling below the critical range (Ar1) temperature. This cycle reduces hardness and reprecipitates the carbide phase as lamellar pearlite. Controlling the time and temperature gives the metallurgist a means to alter the resulting lamellar pearlite structure, and refine the ferritic (as rolled) grain size. 

LP anneals are usually applied to medium carbon (0.40-0.65 weight %) plain carbon and alloy steels for precision machining in order to reduce hardness and improve machinability. 

Spheroidize Anneal   

Spheroidized Microstructure.

Spheroidize annealing is the term that describes a thermal process which results in a globular or spheroidal type of carbide after heating and cooling. There are several types of spheroidize cycles which we will write about in a future post. 

Spheroidized microstructures are desireable for machinability and improved surface finish when machining higher carbon steels. Spheroidized microstructures are also preferred when the steel is to be severely cold worked: cold extruded, cold upsetting, or bent. Most bearing steels are first spheroidize annealed prior to machining. 

Lamellar Pearlite photo 

Spheroidized Photo 

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