In this article we will discuss about:- 1. Meaning of Ausforming 2. Ausforming Process 3. Structural Changes 4. Strengthening Factors 5. Important Applications.
Contents:
- Meaning of Ausforming
- Process of Ausforming
- Structural Changes of Ausforming
- Strengthening Factors of Ausforming
- Important Applications of Ausforming
1. Meaning of Ausforming:
In 1954, two Dutch metallurgists Lips and van Zuilen developed this process of Ausforming and produced thin sheets of 0.35%C-4.5% Ni-1.5% Cr steel with enhanced strength as illustrated and compared with conventional treatment of hardening in table 9.2. In this process, the deformation over 50% of metastable austenite in done in metastable bay between the pearlite and bainite noses of the pertinent TTT diagram (i.e., in range 600-400°C) and is then quenched to martensite (or bainite) as schematically illustrated in Fig. 9.1 (b). It is then tempered.
It is necessary that no transformation occurs when the steel is being deformed. Ausforming (after hardening and tempering) results in higher yield and the tensile strength as compared to conventionally tempered martensite steel without adverse effect on ductility and toughness. Ausforming requires careful control to be successful and also very substantial amount (over 50%) of deformation.
Ausforming Steels:
Ausforming steels should have adequate hardenability to have TTT diagram as illustrated in Fig. 9.1 (b), i.e., deep metastable-austenite-bay between the pearlitic and bainitic noses, and these two noses should be sufficiently displaced to longer transformation times (i.e., towards the right). Also allowance must be kept in mind for the fact that deformation of austenite accelerates the transformation.
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Thus, the steel must have sufficient amount of alloying elements to slow down the reaction and avoid formation of ferrite during cooling from austenitising temperature to the deformation temperature. Most commonly added elements are chromium, molybdenum as these, form deep bays and also elements like nickel and manganese.
Carbon is essential for ausforming, at least a minimum of about 0.05 to 0.10%, but larger additions produce little effect, although steels normally have 0.3 to 0.4% carbon. Strong carbide forming elements such as molybdenum, niobium, vanadium, titanium not only displace the TTT curves towards right, but form a fine dispersion of their carbides which resist softening (coarsening) during tempering and increase the strength of the steels.
2. Process of Ausforming:
1. Austenitising Temperature:
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It should be as low as possible to just dissolve the alloy carbides without causing grain growth. This result in having fine grains of austenite which minimises the martensite plate size on hardening as well as, increases the dislocation density during deformation of metastable austenite.
2. Rate of Cooling Form Austenitising Temperature to Deformation Temperature:
It should be enough to avoid the formation of ferrite during cooling. Even the cooling from deformation temperature to room temperature should be fast enough to avoid the formation of bainite.
3. Temperature of Deformation:
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As the temperature of deformation is lowered, more is the strain hardening of austenite which results in higher strength after ausforming. The temperature should at least be low enough not to allow recovery and recrystallisation. (See Fig. 9.4)
4. Amount of Deformation:
Higher the initial yield strength, the greater is the strengthening during deformation. As the amount of deformation increases, the strength increases almost linearly as illustrated in Fig. 9.3 at the rate of 4 to 9 MPa per percent of deformation.
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In many steels, the ductility actually increases with the increase of deformation particularly above 30% reduction. This probably occurs because the strain-induced precipitation of alloy carbides in the untransformed austenite reduces the carbon content of the martensite formed later.
Another reason put forward is that ausformed martensite is usually twin-free. Twin-intersections are usually sites for nucleation of cracks in BCC metals. Their absence is expected to improve the ductility of steels. Also the martensite plates are fine which could act as crack, and carbides have precipitated uniformly and not concentrated at grain boundaries.
3. Structural Changes of Ausforming:
Ausformed steels have high density of dislocations ~ 1013 cm-2, which have been seen to be present almost uniformly distributed. These dislocations are produced partly during heavy deformation and partly during shear transformation of austenite to martensite.
The following structural changes occur in martensite due to the deformation of metastable austenite prior to the martensite formation:
1. Refinement of the martensite plates, or packets.
2. Increase in dislocation density in martensite. Martensite plate may have inherited fine dislocation substructures from austenite.
3. Change in the size, amount and distribution of carbides.
4. Development of texture in the martensite.
During working, say rolling in ausforming, the austenite grains get elongated in the direction of rolling. This reduced size of austenite transforms to fine plates of martensite in the direction normal to the plane of rolling but may be slightly longer in other directions. This also leads to directional (texture) properties in martensite. Deformation may also cause precipitation of carbides particularly of strong carbide forming elements like molybdenum, vanadium, etc.
These carbides also contribute to refinement of martensite structure by restricting the growth of the martensite plates. This probably occurs because the prior dislocation arrays have been pinned by the fine precipitates and can act as barriers to martensite plate propagation. The martensite is dispersion hardened by these carbides.
4. Strengthening Factors of Ausforming:
As no single factor can explain the high degree of strengthening observed, several factors must be contributing it. Major contribution is due to fine dispersion of alloy carbides associated with dislocations. As carbon free steels do not show much strengthening by ausforming, increased dislocation density as well as increased fineness in plates of martensite has lesser effect in the strengthening of steels. Ausformed steels show more resistance to tempering.
The effect of alloying elements on the response to ausforming is probably based on their effect on stacking fault energy. Nickel raises the stacking fault energy of austenite, and thus reduces strengthening effect of ausforming. Manganese lowers the stacking fault energy, raises the rate of work-hardening and thus, has beneficial effect on ausforming response of the steel. As the finely dispersed carbides of the strong carbide forming elements don’t coarsen easily on tempering, ausformed steels resist softening better than conventionally treated steels.
5. Important Applications of Ausforming:
Ausforming produces some of the strongest, toughest steels obtainable, with very good fatigue properties. These steels are very expensive as these contain expensive elements as well as are to be given heavy reductions needing high powered rolling mills. If cost is not the primary factor, then these steels are very useful for applications which require high strength to weight ratio such as parts for under carriages of air craft, special springs and bolts.
A steel-0.4% C, 6% Mn, 3% Cr, 1.5% Si attains a strength of 3400 MPa with better ductility than obtained by conventional heat treatment. A 12% Cr steel with some additions can attain a strength of 3000 MPa.
Fig. 9.4 illustrates attainment of high strength and ductility in 0.4% C, 5.0% Cr, 1.3% Mo, 1.0% S, 0.5% V steel with good ductility:
If ausforming and strain-ageing are combined then astonishing high levels of strengths (yield as well as tensile) are obtained though ductility decreases drastically, though not entirely lacking (see difference of values for case 3 in table 9.3), but there is a large difference in yield strength and tensile strength as compared to values obtained by conventional heat treatment in 0.6% %, 5.0% Ni steel.