Strain Ageing of Steel: Part Two

요약:

Strain ageing can have a serious detrimental effect on low carbon structural steels and so two material examples are examined to see how different pre-strain and ageing conditions affect material mechanical properties.
A carbon steel (40% martensite) and a microalloyed steel (20% martensite) were both treated under the same parameters and then the UTS and stress strain curves were evaluated to gain some valuable conclusions.

Strain ageing has been found to cause a detrimental effect in low carbon structural steels. A lot of studies have been made on the effect of different parameters on strain ageing characteristics of theses steels.

In the work of S. Gündüz, the ageing behaviour of a carbon steel with 40% martensite volume fractions and a microalloyed steel with 20% martensite volume fractions were studied. The variation of mechanical properties, especially the increase in YS was measured by tensile tests. The specimens were pre-strained in tension by 2, 4 and 6%, aged at 25, 100, 150, 200 and 250°C for 30 min followed by restraining.

The steels used in this investigation are commercially produced a carbon steel (without alloying elements) and a microalloyed steel with a chemical composition shown in Table 1. All the specimens were first subjected to an annealing treatment at 900 °C for 30 min followed by air cooling to homogenize the micro-structure.

Samples used for the annealing treatment are of dimensions approximately 170mm × 30mm × 4.5 mm. A Carbolit furnace capable of operating up to 1200°C was used. The temperature in the heat treatment furnace was measured using a K-type thermocouple and temperature variation during heat treatment did not exceed ±3°C.

  %C %Si %S %P %Mn %V %Ti %Al %Nb
Microalloyed steel 0.11 0.10 0.010 0.020 1.20 0.10 0.05 0.020 0.070
Carbon steel 0.22 0.40 0.020 0.025 1.40 - - 0.015 -

Table 1: Chemical composition of the investigated steels

As mentioned above, the specimens were pre-strained in tension by 2, 4 or 6%. After this, they were unloaded and aged at 25, 100, 150, 200 and 250°C for 30 min. After ageing of the specimens, they were subjected to a tensile test at ambient temperature at a crosshead speed of 2 mm/min. At least three specimens were tensile tested for each ageing temperature and average values were calculated.

The increase in flow stress as a result of restraining was taken as the strain ageing, ΔY2, which is illustrated in Figure 1. For samples, pre-strained in tension, ΔY2 was determined with single specimen by the difference between lower yield stress after ageing and the flow stress at the end of the pre-straining.



Figure 1: Stress–strain curve for low carbon steel strained to point A, unloaded, and then restrained immediately (curve a) and after ageing (curve b).

Figures 2 and 3 show the stress–strain diagrams of the dual phase carbon steel and the microalloyed steel pre-strained in tension by 2, 4 or 6%, aged at different temperatures, and restrained.

As shown, the dual phase carbon steel and the microalloyed steel, prior to any ageing, exhibits continuous yielding which has been commonly attributed to mobile dislocations introduced during cooling from the intercritical annealing temperature. Many dislocation sources come into action at low strain and plastic flow begins simultaneously through the specimen, thereby suppressing discontinuous yielding.



Figure 2: Variation of stress–strain curves of the dual phase carbon steel at different ageing temperatures for the pre-strains of 2% (a), 4% (b) and 6% (c).



Figure 3: Variation of stress–strain curves of the dual phase microalloyed steels at different ageing temperatures for the pre-strains of 2% (a), 4% (b) and 6% (c).

The main conclusions from this study are as follows:
1. Both steels displayed significant changes in appearance as the ageing temperature was increased for the pre-strain in the range studied. This indicated that static strain ageing takes place in both dual phase carbon steel and microalloyed steel.
2. In contrast to the negative effect of pre-strains on the change in ΔY2 produced by subsequent ageing, it was found that increasing pre-strain markedly increased the change of UTS of both dual phase carbon steel and microalloyed steel. This indicated that ΔY2 value is insensitive to dislocation density and is principally dependent on the solute segregation per dislocation.
3. Both dual phase carbon steel and microalloyed steel showed significant increases in YS, UTS and ΔY2, however the percentage elongation to fracture decreased as the ageing temperature was raised from 25 to 100°C for the pre-strain in the range studied. This is due to atmosphere formation at dislocation and precipitation of carbonitride on dislocations during strain ageing.
4. Further increase in ageing temperature to 150, 200 and 250°C caused a reduction in YS, but an increase in percentage elongation. These are signs of overageing probably due to tempering effect of martensite and coarsening of the precipitates on the dislocations.
5. The ageing in the dual phase microalloyed steel occurred more slowly than the dual phase carbon steel. This was associated with the chemical composition of dual phase microalloyed steel which, in addition to carbon atoms, contained nitrogen and carbide forming elements such as titanium, vanadium and aluminum.

기술 자료 검색

검색할 어구를 입력하십시오:

검색 범위

본문
키워드

머릿글
요약

이 문서는 전체 문서 중 일부분입니다. 이 주제에 대해 더 읽고 싶으시면 아래 링크를 클릭하시면 됩니다.

Total Materia Extended Range는 수천개 재질의 탄성 영역 범위 내 계산용 응력-변형률 곡선, 열처리 및 공정 온도 정보를 포함하고 있습니다. 다양한 변형률 속도에 따른 진응력-진변형률 곡선 및 공칭응력-공칭변형률 곡선 또한 제공됩니다.

응력-변형률 곡선을 데이터베이스에서 검색하는 것은 매우 쉽습니다.

신속 검색에 검색할 재질명을 입력합니다. 원하신다면 국가/규격을 지정하신 후 검색 버튼을 클릭합니다.


관심 소재를 선택한 후, 선택된 소재에 대한 응력-변형률 곡선의 링크를 클릭하십시오. 가능한 응력 - 변형률 곡선 기록의 개수는 링크 옆 괄호 안에 표시됩니다.


Total Materia 응력-변형률 곡선은 규격 사양서와 무관하므로, 어떠한 소그룹 내의 링크를 클릭하셔도 응력-변형률 곡선을 검토하실 수 있습니다.

온도가 달라지면 응력-변형률 곡선도 변하기 때문에, 응력과 변형률 정보는 표로 주어져 있습니다. 이는 CAE 소프트웨어 등에 쉽게 응용하실 수 있습니다.


다른 작동 온도에서 응력-변형률 곡선을 검토하실 수도 있습니다.

이를 위해서는 간단히 범위 내 새로운 온도를 온도 입력 창에 입력하시면 됩니다.

계산 버튼을 누르시면 새로운 온도와 그에 대응하는 값이 출력됩니다. 250°C에서의 예를 보십시오.


Total Materia 데이터베이스를 사용해 보실 수 있는 기회가 있습니다. 저희는 Total Materia 무료 체험을 통해 150,000명 이상의 사용자가 이용하고 있는 커뮤니티로 귀하를 초대합니다.