• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.11 s.170
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• pp.1922-1928
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• 1999

Rotary bending fatigue tests were carried out to investigate the influence of artificial defects on fatigue limit in annealed and austempered ductile iron. Obtained main results are as follows : (1) Artificial defect(micro hole type, dia.<0.4 mm) on specimen surface did not bring about a obvious reduction of fatigue limit in austempered ductile iron(ADI) as compared with annealed ductile iron. (2) According to the investigation of $\sqrt{area}_c$ which is the critical defect size to crack initiation at artificial defect, $\sqrt{area}_c$ of ADI is larger than that of annealed ductile iron. This shows that the situation of crack initiation at artificial defect in ADI is more difficult in comparison with annealed ductile iron. (3) One of the reasons for the low rate of crack initiation from artificial defect in ADI is that the resistance of matrix to crack initiation is higher than that of annealed ductile iron. (4) In case that the $\sqrt{area}$ of artificial defect and graphite nodule is the same, the rate of crack initiation from graphite nodule is higher than that from artificial defect. This reason is that the serious ruggedness around graphite nodule is formed by austempering treatment.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.1
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• pp.46-53
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• 1998

The variation of the mechanical properties, microstructures and the formation of retained austenite with heat treatment conditions in austempered ductile cast iron has been investigated. In the case of austempered ductile cast iron below 25mm diameter, it has been found that a pearlite structure are not obtained under a super cooled condition at range of $0.05^{\circ}C/sec{\sim}10^{\circ}C/sec$, and the matrix is precipitated in graphite, bainite and retained austenite. After austempering treatment the retained austenite is increased with decreasing cooling rate. The elongation increases with decreasing super cooling rate, and the optimum result has been shown to be the elongation of 15.6% at super cooling rate of $0.05^{\circ}C/sec$. The optimum result has been shown to be the tensile strength-elongation balance of $1656kgf/mm^2.%$ and it is more than doubled to as the casting state and continuous cooling condition.

• Journal of Korea Foundry Society
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• v.12 no.5
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• pp.403-411
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• 1992

The growth characteristics of bainite at early stage in the fast quenched spheroidal graphite cast irons containing 0.06%Mn and 0.45%Mn during austempering process, was investigated with optical and scanning electron microscope. The following results regarding the effects of Mn and isothermal heat treatment on the morphological variation of bainitic ferrite were obtained. The morphology of bainite varies from acicular below 350$^{\circ}C$ to feather shape above 350$^{\circ}C$. The period of isothermal treatment also affects the shape of bainite at the fixed temperature. At 350$^{\circ}C$, bainite is bamboo leaf-like up to 200 secs of isothermal holding time and with further increasing time up to 300 secs, changes to a mixed structure consisting of both feather and bamboo leaf and, finally becomes all feather shape at 900 secs. The morphology of bainitic ferrite formed at early stage of 300$^{\circ}C$ isothermal treatment is similar to that of bainitic ferrite formed at 250$^{\circ}C$ or 350$^{\circ}C$ with unbranched, linear ferrite. However, bainitic ferrite divides into branches with increasing isothermal treatment, which occurs more fast at 400$^{\circ}C$ than at 350$^{\circ}C$. The difference in adding amount of Mn influences the morphology of bainitic ferrite in upper bainite. The bainitic ferrite with 0.45%Mn is observed to be more stable than that with 0.06%Mn, remaining unbranched for a longer period at the same temperature.

• Journal of Korea Foundry Society
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• v.25 no.3
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• pp.123-127
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• 2005

0.8wt.% C과 $0.75{\sim}3.2$(0.75, 1, 1.26, 1.44, 1.89, 2.1, 2.46, 2.94, 3.22)wt.% Si을 함유한 고탄소 고실리콘 강에서 Si의 함량이 미세조직과 processing window에 미치는 영향에 대해 조사하였다. 열처리는 $940^{\circ}C$에서 1시간 동안 오스테나이징을 실시한 후, $320^{\circ}C$에서 $0.2{\sim}300$분 동안 오스템퍼링을 하였다. 미세조직에서, 0.75 wt.% Si을 함유한 강에서 국부적으로 오스페라이트(ausferrite)가 존재하는 것을 확인하였다. Processing window는 경도 및 XRD 측정으로 결정하였다. $t_{1}$은 Si의 함량이 2.5 wt.% 이하에서는 거의 변화가 없다가,2.5 wt.% 이상에서는 감소하는 결과를 보였다. 반면, $t_{2}$는 Si의 함량이 증가함에 따라 연속적으로 증가하였다. 즉, 0.8 wt.% C을 함유한 강에서 processing window는 Si의 함량이 약 2.5 wt.%일 때 열렸다.

• Journal of the Korean Society for Precision Engineering
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• v.15 no.11
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• pp.32-38
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• 1998

For the purpose of monitoring the abnormal state in proportion to cutting in automatic production process, the 3 kinds of specimens different from mechanical properties by austempering through temperature variation were manufactured, and the effects of tool wear on thrust and AE RMS was analyzed with sequential drilling in in-process. When the ADI specimens were drilled, the relationship of thrust and AE RMS with flank wear was studied through experiments, and it is confirmed that the reliable wear state is able to be monitored by using these signals. It was shown that thrust and AE RMS increased slowly till flank wear reached to V$_{B}$ = 0.25mm, and they increased steeply over the value. The effective tool exchange time was able to be pre-estimated by using this fact. It was validated that the tool breakage was able to be detected on the real time by monitoring in in-process.s.

• Journal of Korea Foundry Society
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• v.10 no.4
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• pp.299-308
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• 1990

Ductile cast iron has a good ductility and ductility and toughness than those of gray cast iron, because the shape of graphite is spheroidal. It has been reported that the strengthening and toughening of the ductile cast iron was resulted from the good modification of various matrix structures obtained by the heat treatment or addition of alloying elements. This study aims to investigate the effect of various special heat treatment(Cyclic Heat Treatment, Intermediate Heat Treatment, Step Quenching), austempering and alloying element(Ni) on the strength and toughness of ductile cast iron. The results obtained from this study are summarized as follows. 1) With addition of Ni, the amount of pearlite or bainite were increased and the morphologies of pearlite or bainite became finer by special heat treatments. 2) As the Ni added and not added ductile cast iron were treated by austenitizing at $900^{\circ}C$ and $840^{\circ}C$, in the latter the austenite was mostly formed in the vicinity of eutectic cell boundary, but in the former on the whole matrix. 3) In cyclic heat treatment, the volume fraction of pearlite was increased and the shape of pearlite was fined with increase of the number of cycle. 4) The shape of pearlite was mostly bar type in the intermediate heat treatment, but spheroidal type in step quenching. 5) The mechanical properties of ductile cast iron containing 1.5%Ni austempered at $400^{\circ}C$ for 25min. after austenitizing at $900^{\circ}C$ for 15min. were a good value of hardness 105(HRB), impact energy 12.5(kg.m), tensile strength 112($kg/mm^2$) and elongation 6.8(%).

• Journal of Korea Foundry Society
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• v.32 no.2
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• pp.104-108
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• 2012

The effect of heat treatment on mechanical properties of 0.4C-2.3Si(wt%) steel with bainitic ferrite matrix were investigated. This steel has been synthesized intergrating concepts from TRIP(Transformation Induced Plasticity) steel & Austempered Ductile Cast Iron(ADI) technology. The low alloy medium carbon (0.4 %C) steel with high silicon (2.3 %Si) was initially annealed for 60 min at $800^{\circ}C$, $820^{\circ}C$ and $840^{\circ}C$ respectively in the intercritical region and then subsequently austempered at various temperatures at $260^{\circ}C$, $320^{\circ}C$ and $380^{\circ}C$ for 30 min in a salt bath. The mechanical properties were measured by using a tensile test. A detailed study of the microstructure of this steel after heat treatment was carried out by means of electron back scattering diffraction (EBSD) technic. In this study, a new low alloy steel with high strength (780~1,050MPa) and exceptionally high ductility (20~40%) was obtained.

• Korean Journal of Materials Research
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• v.19 no.6
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• pp.305-312
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• 2009

The effects of Quenching and Partitioning (Q&P) and Annealed Martensite (AM) heat treatment on the microstructure and tensile properties were investigated for 0.24C-0.5Si-1.5Mn-1Al steels. The Q&P steels were annealed at a single phase ($\gamma$) or a dual phase (${\gamma}+{\alpha}$), followed by quenching to a temperature between $M_s$ and $M_f$. Then, enriching carbon was conducted to stabilize the austenite through the partitioning, followed by water quenching. The AM steels were intercritically annealed at a dual phase (${\gamma}+{\alpha}$) temperature and austempered at $M_s$ and $M_s{\pm}50^{\circ}C$, followed by cooling in oil quenching. The dual phase Q&P steels showed lower tensile strength and yieldyield strength than those of the single phase Q&P steels, and tThe elongation for the dual phase Q&P steel was partitioning 100s higher than that of that for the single phase Q&P steels as the partitioning time was less than 100s up to partitioning 100s. For AM steels, the tensile/yield strength decreased and the total elongation increased as the austempering temperature increased. The stability of the retained austenite controlled the elongation for Q&P steels and the volume fraction of the retained austenite controlled the elongation for AM steels.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.1
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• pp.1-9
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• 1997

The variation of the mechanical properties and the formation of retained austenite with heat treatment conditions in austempered Si bearing carbon steels has been investigated. In the case of a steel containing 0.35C-1.48Si-0.95Mn, it has been found that a feather shape bainite structure of lath are obtained under a isothermal treated condition at just below the Ms temperature, and the martensite, bainitic ferrite and retained austenite of second phase particles on the ferrite matrix for a isothermal treated steels after intercritical annealing are precipitated in a linked shape. The retained austenite with $2{\mu}m$ size induced as TRIP is found to increase with increasing the formation rate of retained austenite for the intercritical annealing and high Si containing steels. The tensile strength is increased as austempering temperature increases in all isothermal treatment temperature, whereas the elongation is shown to roughly decrease as the tensile strength increases. The values of tensile strength-elongation balance have showed a marked dependence upon the elongation rather than the tensile stregth, and their values are increased for high Si containing steels and intercritical annealing condition. The most optimum result has been shown to be the tensile stregth-elongation balance of $2882.4kgf/mm^2.%$ and the elongation of 33.3% for a "B" steel in the heat treating temperature range of $780{\sim}370^{\circ}C$.

• Journal of Korea Foundry Society
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• v.41 no.6
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• pp.543-549
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• 2021

Aluminum cast iron has excellent oxidation resistance and good resistance to sulfide and corrosion. Compared to Ti and Ni alloys, it is expected to be a substitute material for structural materials and stainless steels because it is relatively inexpensive to use Fe, which is a non-strategic element. This results in a weight reduction effect of about 30% as compared to the use of stainless steel. With regard to aluminum as an alloying material, it is an element that has been widely used for the alloying of cast iron in recent years. Practical use has been delayed owing to the resulting lack of ductility at room temperature and the sharp decrease in the strength above 600℃ of this alloy, however. The cause of the weak room temperature ductility is known to be environmental embrittlement by hydrogen, and the addition of various alloying elements has been attempted in order to mitigate these shortcomings. Although alloying elements such as vanadium, chromium, and manganese are mainly used to increase the hardness and wear resistance of gray cast iron, the price of finished products containing these elements and the problems associated with alloys with this material impose many limitations.