4 Reasons to Consider Normalizing Steel

October 24, 2013

Normalizing is a thermal process where  steel is heated about 100-150 degrees F  above the critical range followed by cooling in still air to below that range.

Not a fan of expensive thermal treatments without a good reason...

Not a fan of expensive thermal treatments without a good reason…

On some steels, this normalize process is followed by a temper or stress relief anneal below the Ac1 to remove residual stresses resulting from the air cooling and to reduce hardness.

 Normalizing Steel gives you

  • Reduced hardness and removes residual stress
  • Improves machinability
  • Develops desired mechanical properties (especially in larger sections)
  • Improved austenitizing  for subsequent quench and tempering

Adding costly thermal treatments to a production process is seldom a good idea. But

  • if you need high side mechanical properties as a result of the quench and tempering operation you have planned,
  • if the heat is lean on chemistry,
  • if the part to be quenched has a large cross section or wall thickness,
  • if you know from experience that you have difficulty getting to high side with your quench,

Normalizing can help.

For the end quench position of the bar that corresponds to 90% martensite,  a non-normalized  4140,  austenitized at 1550 degrees can have anywhere from a 7 to 14 point Rockwell C hardness deficiency compared to the same steel that had been normalized.  Using an austenitizing temperature of 1650 (200 degrees F above the Ac3 temperature) the non-normalized 4140 could still exhibit as much as a 10 Rc hardness deficiency compared to normalized stock for the same time at temperature. For 4340 steel, the hardness deficit can  range from 10 to 18 points of Rockwell C hardness deficiency for the same austenitizing time.

Normalizing was a necessary step in the days of highly variable microstructures resulting from Ingot steels and analog controlled processes. Today’s modern computer controlled steel making processes provide more consistent products and structures, making normalizing a less utilized process. But normalizing remains a way to coax better properties or performance out of some steels.

We’re not a big fan of adding “fire for fire’s sake,” but if you suspect you may have difficulty in developing the full hardenability out of your steel, Normalizing may just give you the edge you need to assure you develop the as quenched hardness you need.

Photo credit: Above The Law Blog


Martensite- Five Facts

March 15, 2011

1) Martensite is the hardest and most brittle microstructure obtainable in a given steel.

2) Martensite hardness of the steel is a function of the carbon content in that steel.

3) Martensite results from cooling from austenitic temperatures rapidly by pulling the heat out using a liquid quenchant before pearlite can form.

4) As quenched Martensitic structures are too brittle for economic use-they must be tempered.

5) Reheating as quenched Martensite to a temperature just below the AC1 results in the best combinations of strength and toughness.

This is what you get when you cool faster than the critical cooling (pearlite transition) rate- Martensite

 

Hardness of martensite is a function of carbon content

 

Softening of martensite in 0.35%C, 0.8% C, and 1.2% C carbon steels by tempering at the indicated temperature for 1 hour.

Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase, unlike ferrite and pearlite which result  from a slower chemical diffusion process, so each have different chemical compositions than the parent austenite.

Formation of Martensite involves a transformation from a body-centered cubic structure to  body-centered tetragonal structure. The large increase in volume that results  creates a highly stressed structure. This is why Martensite has a higher hardness than Austenite for the exact same chemistry…

Photo  and Graphs Credit: Cold Finished Steel Bar Handbook


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