Contribution of Fracture Mechanics to Material Design: Part Four

요약:

An example of the design of a new steel type is steel with intermetallic phases. This steel contains needle-like martensite laths, smaller than 1μm, which provide high yield strength and good toughness. Further improvements of the properties are not possible by additional alloying with carbon or nitrogen. Since the needle-like martensite can be obtained only in very low carbon content, further increase of yield strength is possible only by controlled precipitation of intermetallics.

Modern Trends

Can steel be further improved? Yes, of course. Two main directions are: (1) design of new types of steels for particular applications (physical metallurgy/fracture mechanics approach) and (2) improvement of technology (technological approach).

An example of the design of a new steel type is steel with intermetallic phases. This steel contains needle-like martensite laths, smaller than 1μm, which provide high yield strength and good toughness. Further improvements of the properties are not possible by additional alloying with carbon or nitrogen. Since the needle-like martensite can be obtained only in very low carbon content, further increase of yield strength is possible only by controlled precipitation of intermetallics. The measures for technological improvement are:

  • Cleanness of steel: The state of art of steel making technology can produce very clean steel in terms of N+O+S+P<50ppm! Clean steel improves toughness of both base metal and HAZ.
  • Inclusion shape control: Even though S content is lowered to the level of 10 ppm, it is not possible to avoid formation of MnS in the central segregation zone. As pointed out previously, MnS tends to elongate, behave as a stress concentrator and lowers the toughness.
  • Centerline segregation in slab: The continuously cast slab always has a centerline segregation, characterized with higher concentration of Mn, C, P and S. The intensity of centerline segregation can be reduced by having cleaner steel, by the reduction of slab thickness during steel casting and by accelerated cooling, or by combination of these processes.
  • Slab reheating temperature: Reheating temperature in furnace must provide both homogenous grain size and dissolution of alloying elements.
  • Accelerated cooling increases the undercooling and nucleation rate, enabling additional refinement. One of results is the absence of clear yield point, due to bainitic transformation.

The development of steel as a structural material is summarized in Figures 6 and 7.

Figure 6: Schematic interrelationship between the yield strength and the transition temperature T27 for different steel types.

Figure 7: Change of carbon content in structural steels during their development.

It is worth noting that each new generation of steel follows the direction to both decreasing transition temperature and increase yield point. Also, this development have been accompanied with improvements in weldability, formability etc. Figure 7 shows that development of steels for pipelines had to be accompanied with improvements in weldability, therefore, the carbon content is very low, while good toughness and high yield point are achieved by complex alloying (combination of Nb, V and Ti).

Another example of the contribution of fracture mechanics will be described on development of medium carbon microalloyed steels for sucker rods in oil industry. These rods are being subjected to heavy dynamic loading in a very aggressive atmosphere. Therefore, the requirements are very high.

It is known that the presence of acicular ferrite considerably improves toughness, due to the shape of ferrite grains, and that bainite brings corrosion resistance. Therefore, a new steel containing 0.03% C; 0.33% Si; 1.5% Mn; 0.1% V; 0.012% N; and 0.01% Ti has been designed for this application. Rods are bonded by screws, therefore the absence of welding have allowed relatively high carbon content. Effect of temperature on CVN Impact Energy and microstructures is illustrated in figure 8. The steel has been produced by hot forging with subsequent cooling on still air.

Figure 8: The effect of temperature on CVN Impact Energy of medium carbon V-microalloyed steel.

Figure 9: Microstructure of tested steel with MnS-VN inclusion; INI = intragranularly nucleated ferrite.

Very low transition temperature, in spite this is not a Q+T steel, is obtained due to the dominant presence of acicular ferrite in the microstructure. It has been suggested that non-metallic particles are necessary for nucleation of acicular ferrite. The extent of nucleation depends on composition, crystal structure, number, size and interparticle distance etc. Second condition is grain size on annealing temperature, i.e. grain should have some optimal size, rather larger than smaller, because larger grains will decrease temperature of austenite decomposition to the temperature range in which acicular ferrite is a dominant structure.

Hardenability has a similar role. Second phase particles are usually oxides and/or nitrides or sulfides; MnS particles served as preferential places for precipitation of VN, which has a great potential for nucleation of intragranular proeutectoid ferrite (intragranularly nucleated ferrite – INI), which in turn serves as a nucleation site for acicular ferrite. Therefore, even considerably high content of sulfur (130 ppm) hasn’t damaged toughness, i.e. nucleation of acicular ferrite wouldn’t be possible without MnS particles. This conclusion is very important for further practical design of materials.

Based on the advancement of fracture mechanics, consideration of particles in steel is not only in direction of smaller content of impurities, but more to control the shape and distribution of second phase particles. It is much easier and reasonably cheaper to control the shape of inclusions, than to produce steel with very low sulfur content. If the shape of second phase is modified into sphere, than no stress concentration will occur, and presence of inclusions can be sometimes neglected or even beneficial!

Conclusion

Fracture mechanics has introduced the relation between material properties, service conditions and defects in the structure originating from fabrication. In the field of materials design, fracture mechanics has improved the quantification of both shape and size of inclusion/second phase particles, allowing less conservative approach to design, i.e. practical dealing in materials production with an aim to produce material with acceptable impurity content distributed in controlled shapes and sizes. This approach has opened a whole new area both in the materials design and methods for detecting defects in materials.

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 금속 합금의 열처리 조건 및 하중 조건에 대한 파괴 공학 변수 데이터베이스를 포함하고 있습니다. K1C, KC, 균열 성장 및 Paris 법칙 변수와 이에 대응되는 균열 성장 그래프가 제공됩니다.

단조 특성도 참조를 위해 추가되어 있으며 단조 특성을 통해 예상되는 빠진 변수의 추정치도 필요한 곳에는 포함되어 있습니다.

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


결과 리스트에서 재질을 선택하시면, 일련의 규격 사양 소그룹이 나타납니다.


Total Materia 파괴 공학 데이터는 규격 사양서와 무관하므로, 어떠한 소그룹 내의 링크를 클릭하셔도 파괴 공학 데이터를 검토하실 수 있습니다.

데이터는 표로 출력되며, 가능한 곳에는 Paris 곡선(Region II)와 함께 출력되어 있습니다. 데이터 출처에 관한 정확한 참조가 각 데이터에 주어져 있습니다.


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