One Minute Mentor: Austenitizing Tool Steel

October 16, 2015
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Austenitizing Tool Steel

Austenitizing is the heat treatment in which the final alloy element partitioning between the austenitic matrix (which will transform to martensite) and the retained carbides occurs. This partitioning fixes the chemistry, volume fraction, and dispersion of the retained carbides. The retained alloy carbides not only contribute to wear resistance but also control austenitic grain size. The finer the carbides and the larger the volume fraction of carbides, the more effectively austenitic grain growth is controlled. Thus, if the tool steel is austenitized at too high a temperature, undesirable grain growth may occur as the alloy carbides increasingly coarsen or dissolve into the austenite.

The graph shows that tempering by a combination of retained austenite transformation and secondary hardening, will eventually raise the hardness of as-quenched structures with large amounts of retained austenite. Not shown is the deleterious increase in austenite grain size that develops as more and more carbides dissolve at the higher austenitizing temperatures.

Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Carbon and Low-Alloy Steels ->  Introduction to Heat Treating of Tool Steels -> Heat Treatment Applied to Tool Steels

One Minute Mentor: Sinter Hardening

September 24, 2015
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Sinter Hardening

Sinter hardening refers to a process in which the cooling rate in the cooling zone of the sintering furnace is fast enough to transform a significant portion of the powder metal steel matrix into martensite.

The sinter hardening route has become increasingly widespread due to its cost effectiveness. Advantages include:

• ·               Eliminating secondary quench hardening treatment
  ·               Better dimensional control due to less severe quench
  ·               Elimination of retained quench oil in pores leading to fewer steps during tempering
  ·               Oil does not have to be removed to perform secondary operations such as plating

• Disadvantages of sinter hardening include:

• ·               Difficult or impossible machining due to high hardness
  ·               Need for tempering operation to relieve stresses and improve dynamic mechanical properties
  ·               Impractical sizing or other secondary operations due to the high apparent hardness

• Vacuum sinter hardening and gas quenching shows some promise as another method to achieve higher hardness. A comparison of vacuum sinter-hardened materials to wrought 4600 steel is shown in the graph. PM steels were sintered for 2 hours at 1260°C (2300°F) in a hydrogen atmosphere, followed by a 2-bar helium gas quench. The time at each temperature is one hour. As the graph shows, hardness is comparable to the wrought equivalent.

Volume 4D, Heat Treating of Irons and Steels â‡’ Heat Treating of Carbon and Low-Alloy Steels â‡’  Heat Treatment of Powder Metallurgy Steels â‡’  Sinter Hardening

One Minute Mentor: Tempering Powder Metallurgy Steels

September 18, 2015
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Tempering Powder Metallurgy Steels

Usually, powder metallurgy steel parts having a density greater than 6.7 g/cm³ (0.242 lb/in.³) should be tempered after hardening. Recommended tempering temperatures for PM parts range from 150 to 200°C (300 to 390°F) with 175°C (350°F) being common. The effect of tempering temperature on impact properties is shown in  the graph for FL-4205 at various densities.

Tempering above 200°C results in improved toughness and fatigue properties of the hardened part, while making a compromise in tensile strength and impact resistance. However, when tempering at temperatures higher than 200°C, care should be exercised as entrapped oil can ignite.

Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Carbon and Low-Alloy Steels ->  Heat Treatment of Powder Metallurgy Steels -> Quenching and tempering