Austempered steel

Ausferritic and bainitic steel as winning solutions compared to traditional quenched and tempered steels

Austempering of steel: the efficient solution for improving the properties of steel and reducing treatment costs

Does it make sense to distinguish between the two types of austempered steel, ausferritic versus bainitic? Both have been developed as alternatives to the quenching and tempering of steel. Let’s take a look at the differences between them.

The first phase of heat treatment for steel is austenization as it is for spheroidal graphite iron. Unlike spheroidal graphite iron, however, during the austenization of steel, the carbon content in the matrix is exactly the same as the chemical composition of the steel. In spheroidal graphite iron, on the other hand, the carbon in the matrix is aligned with the saturation curve at the austenization temperature (Fe-C diagram). Silicon plays a key role between ausferritic and bainitic steel.

Ausferritic steel

A similar effect to the one observed in the matrix of graphite spheroidal iron can be obtained in steel only on the outer surface through cementation. If a steel component is maintained at an isothermal temperature in a salt bath (at a temperature greater than the Martensite Start – Ms), a reaction occurs that is similar to the one for graphite spheroidal iron, but only if the steel contains a sufficient quantity of silicon (e.g. 1.6-2% as in 60Si7 steel). The silicon increases the mobility of carbon during austempering, thus avoiding its chemical combination with the iron, i.e. it allows the carbon-enriched austenite lattice to be stabilised, preventing the formation of carbides. This allows an ausferritic structure to be obtained in the steel as well (acicular ferrite and carbon-stabilised austenite), although with a lower stability than that of graphite spheroidal iron due to the smaller carbon content available in the matrix.

500x

Ausferritic steel is very useful in components that require good wear resistance, excellent toughness and impact resistance, such as agricultural tools for working uneven, stony ground (e.g. used for agricultural implements, blades, springs, leaf springs).

Static and fatigue properties

Zanardi Fonderie experimental data taken from austempered steel 56SiCr7 (ausferritic)

Acciaio austemperato 56SiCr7
Rm [N/mm²]
Rp 0,2 [N/mm²]
As [%]
HRc range
Valori guida
Kv 23°C
[J]
σA50% r.b liscio
[Mpa]
NATURALE
1080
650
11
33
5
600
BONIFICATO
1600
1350
8
50
8
630
AUSTEMPERATO (ausferritico)
1500
1200
9
50
30
650
BENCHMARK 42CrMo4 BONIFICATO
1100
850
14
35
60
440

Thickness [mm] t ≤ 25 mm
[1] Fatigue properties obtained from rotating bending (R=-1) on machined samples Ø6.5 mm obtained from Lynchburg test specimens obtained from Type I separately cast samples according to the ISO17804 standard. Fatigue test configuration according to ISO1143; end life estimation according to ASTM E739-91 and Hertzberg “q”; fatigue limit obtained with the Brownlee-Hodges-Rosenblatt short staircase method.
[2] Fatigue properties obtained from single casting gear samples. Fatigue test according to ISO6336.

The austempering heat treatment has caused the formation of ultra-fine microstructures resulting in increases up to 180% of material strength at the same elongation at break compared to the same steel in the natural state and a significant increase in resilience to room temperature up to 600% compared to the material in the natural state, also observed at the lowest test temperatures (-20 and -40 °C). The data obtained show that steel with a high silicon content can be subjected to austempering treatment, thus increasing both its strength and its ductility and toughness.

What is bainite and how does it differ from ausferrite?

If there is not enough silicon in the steel to ensure good carbon mobility, the carbon will be forced to combine chemically with the iron, thus forming carbides. The fine, uniform mixture of acicular ferrite and lamellar-type carbides is called bainite, named after its discoverer (Bain), who was unable to distinguish the two phases separately because he did not have high resolution microscopes. Bainite has a chemical composition similar to pearlite, but differs in the smaller size of its constituent phases.

Bainitic steel

Bainitic steel is used in components which require good wear resistance combined with fatigue strength, such as mower blades, clips and clamps, cutting and mixing blades, transmission gears, corrugated plates and spikes for lawn aeration (soles, shoes, agricultural implements).

500x

Acciaio austemperato 27MnB5
Rm [N/mm²]
Rp 0,2 [N/mm²]
As [%]
HRc range
Valori guida
Kv 23°C
[J]
σA50% r.b liscio
[Mpa]
NATURALE
800
6550
14
25
15
360
BONIFICATO
1100
750
14
30
40
-
AUSTEMPERATO (bainitico-martensitico)
1200
1000
12
40
60
430
BENCHMARK 42CrMo4 BONIFICATO
1100
850
14
35
60
440

Thickness [mm] t ≤ 25 mm
[1] Fatigue properties obtained from rotating bending (R=-1) on machined samples Ø6.5 mm obtained from Lynchburg test specimens obtained from Type I separately cast samples according to the ISO17804 standard. Fatigue test configuration according to ISO1143; end life estimation according to ASTM E739-91 and Hertzberg “q”; fatigue limit obtained with the Brownlee-Hodges-Rosenblatt short staircase method.
[2] Fatigue properties obtained from single casting gear samples. Fatigue test according to ISO6336.

The study confirmed the possibility of obtaining a structure with high strength and toughness for 27MnCrB5 steel that underwent austempering heat treatment. The data obtained show that the samples undergoing bainitic tempering feature better results in terms of flow stress, tensile strength, hardness and resilience compared to the samples that were quenched and tempered. Compared to the same material that was quenched and tempered, it was possible to increase:

  • up to 30% in ultimate tensile strength
  • up to 50% in resilience both at low and room temperature
  • with a modest reduction in elongation at break

In conclusion

For both types of austempered steel – ausferritic and bainitic – austempering can be used to replace the traditional treatment of quenching and tempering while obtaining an increase in the basic material properties and a reduction in treatment costs.

Other materials heat-treated in subfurniturara by Zanardi Austempering

Discover the material

ADI

Discover the material

IDI

Discover the material

AGI

Materials produced by Zanardi Foundries

Discover the material

ADI

Discover the material

IDI

Discover the material

GJS

Case History

Real cases explaining the development of client projects