Relevant aspects of allotriomorphic and idiomorphic ferrite transformation kinetics

Capdevila, Carlos; Caballero, Francisca G.; Andrés, Carlos García de

doi:10.1179/026708303225008671

Cited by 8 publications

(11 citation statements)

References 23 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…ISIJ International, Vol. 48 (2008) But in 5B steel V(C, N) precipitation has been greatly eased by the high N content, which increases the ratio between intragranular and grain boundary nucleation sites, 28,29) this together with the annihilation of bainite nucleation sites by the presence of proeutectoid ferrite, provide with the ideal conditions for AF formation, see Figs. 7(b) and 7(c), transcending the unfavourable situation given by a small PAGS at T g ϭ975°C.…”

mentioning

confidence: 99%

Influence of V Precipitates on Acicular Ferrite Transformation Part 1: The Role of Nitrogen

et al. 2008

View full text Add to dashboard Cite

This paper (part 1 of a two part study) deals with the influence of N in its combination with V, as V(C, N) precipitates, on the decomposition of austenite into acicular ferrite. Likewise, the intragranular nucleation potency of V(C, N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C-Mn-V steels with different contents of N under two different austenitising temperatures, i.e. different austenite grain sizes. The results clearly show that for austenite to decompose into acicular ferrite is necessary, first to have a representative fraction of V(C, N) precipitates within austenite, and second to decorate the austenite grain boundaries with proeutectoid ferrite so bainite can not form. Part 2 of the study concerns with the influence that those precipitates have on the kinetics of acicular ferrite formation during austenite isothermal decomposition.KEY WORDS: acicular ferrite; bainite; V(C, N); continuous cooling transformation diagrams.potent inclusions/precipitates within austenite for AF formation.Given that bainite and AF might be competitive microstructures in the temperature range between B S and M S , the different ways to switch between one or the other are described as follows. To promote AF formation the first method relies on reducing the AGB area per unit volume, that is increasing the austenite grain size. Since bainite nucleates intergranularly, bigger grain sizes will slow down the kinetics and favour intragranular nucleation, also by increasing the grain size the probability of trapping more inclusions/precipitates within austenite is also intensified. In the second method the aim is to dishabilitate the bainite nucleation sites, i.e. AGB. This is achieved by decorating the grain boundary with fine proeutectoid ferrite.15) It has also been postulated that B or Mn segregation to the g-g grain boundaries reduces the driving force for bainite nucleation at the interface.15) Finally, the last and more obvious method is related to the presence of potent inclusions or precipitates within the g grain, if there are none, acicular ferrite formation will never occur.It has been shown that V(C, N) precipitates associated with MnS,16) or introduced in the steel as sintered polycrystal ceramics, 17) or obtained during austenite deformation 18) are responsible of intragranular ferrite nucleation. In this work the influence of N on the anisothermal decomposition of austenite in two V alloyed steels is analyzed. The aim is to elucidate the potency of V(C, N) precipitates for the nucleation of AF. To facilitate this, continuous cooling transformation (CCT) diagrams under different austenitising conditions of two experimental steels containing V in the same quantity but with and nearly without N have been studied. Material and Experimental TechniquesThe chemical composition of the two laboratory grades used in this work are listed in Table 1. Both are low C-Mn-V steels with traces of other elements, the N content was intentionally different, thus 4B steel has a low N leve...

show abstract

mentioning

confidence: 99%

Influence of V Precipitates on Acicular Ferrite Transformation Part 1: The Role of Nitrogen

et al. 2008

View full text Add to dashboard Cite

show abstract

“…such a big grain size is expected to enhance AF formation in detriment of bainite by increasing the ratio between intragranular and grain boundary nucleation sites. 6,7) In the case of the Route 1 the necessary thermodynamic calculations to estimate the B S temperature according to Bhadeshia's theory 8) were performed by means of Ref. 4), and the obtained result was 590°C.…”

Section: Resultsmentioning

confidence: 99%

Influence of V Precipitates on Acicular Ferrite Transformation Part 2: Transformation Kinetics

et al. 2008

View full text Add to dashboard Cite

A combination of thermodynamic models as well as some physical metallurgical principles has been used to analyse the influence that V(C, N) precipitates have on the transformation kinetics of acicular ferrite by isothermal decomposition of austenite. Those precipitates were found absolutely necessary for the nucleation of acicular ferrite as reported in Part 1 of this work, now in Part 2, through the proper design of a heat treatment route we have studied the effect that those precipitates have on the kinetics of acicular ferrite transformation.

show abstract

“…A cicular ferrite does not grow in sheaves because their development is stifled by impingement between plates nucleated independently at adjacent sites [49,138].The larger prior austenite grain diameter (PAGD), as shown in Fig. 2.45 enhances the transformation austenite-to-acicular ferrite in detriment to reconstructive and bainite transformation [49,94,136,139,140,141,142,143,144]. The large austenite grains are required for the preferential formation of acicular ferrite relative to bainite [15,49,94,130].…”

Section: Fig 238 Variation In Mechanical Proerties Of Haz As Functimentioning

confidence: 99%

“…Results are given in Table 4 (ii) The grain size in both steels is large enough to enhance AF formation instead of bainite by increasing the ratio between intragranular and grain boundary nucleation sites [142,143].…”

Section: Alloying Elements Distributionmentioning

confidence: 99%

“…The austenite grain size control is important factor in development of the final mechanical properties of the product [129].Previous results showed that prior austenite grain size of approx 60μm provides optimal conditions for formation of acicular ferrite [142,143]. In that respect, samples were reheated at 950, 1050, 1100, 1150, and 1250 °C.…”

Section: Reheating Temperaturementioning

confidence: 99%

See 1 more Smart Citation

Austenite decomposition in medium carbon microalloyed steels: Mechanism, structure and properties

Fadel¹

View full text Add to dashboard Cite

The aim of this work was to determine TTT diagrams of two mediumcarbon V microalloyed steels V-N (0.256%C, 0.0235%N, 0%Ti) and Ti-V-N (0.309%C, 0.221%N, 0.011%Ti ). The isothermal treatment was carried out at 350, 400, 450, 500, 550 and 600 ο C. These treatments were interrupted at different times in order to analyze the evolution of the microstructure.Isothermal decomposition of medium carbon vanadium microalloyed austenite was evaluated by optical and SEM metallography.In the first step, austenite grain size is establish in temperature range 850-1150 °C. Ti bearing steel exibits lower grain size at high temperatures. This effect is attributed to pinning effect of TiN particless. Nevertheless, temperature of 1100°C provided grain size very simillar to 60μm, value suggested as optimal for intragranular nucleation and formation of acicular ferrite.Isothermal treatment enabled plotting the TTT diagrams. In both diagrams, equal transformations are observed. Influence of Ti addition is not very clear; it is assumed that increased level on carbon has covered expected influence on intragranular nucleation. Four curves are found to be relevant for austenite decomposition in medium carbon Vmicroalloyed steels:(1) Grain boundary ferrite is the first phase to be generated at all temepratures. In the lower temperature range the widmanstetten ferrite is formed, while on higher temperatures grain boundary allotriomorphs are produced. This difference is attributed to displacive nature of transformation at lower and diffusional transformation at higher temperatures.(2) Second curve is related to nucleation of Intragranular ferrite (IGF). In the lower temperature range (350-400°C ) acicular ferrite plates are grouped in sheaves; at intermediate temperatures (450-500°C), a more interlocked microstructure of acicular ferrite was clearly observed, while microstructure generated at high temperatures (550-600°C) is characterized by polygonal idiomorphic ferrite.(3) Third curve is related to onset of pearlite. It occurs at temperatures ≥ 500°C, followed by an incomplete reaction phenomenon.vi (4) The transition between an acicular ferrite sheaf morphology and interlocked microstructure is observed to take place at 400/450°C. However the bainitic sheaves are frequently observed when the isothermal transformation time is increased at 400°C and temperature diminishes to 350°C.Finish of transformation was clearly observed at temperatures below 500°C. However at 550and 600°C, incomplete reaction phenomenon occurs. This behaviour is attributed to carbon enrichment in austenite and decrease of driving force for austenite decomposition.Martensite start temperature is established to be 320°C and 330°C for Ti-V-N and V-N steels respectively. Slightly lower Ms temperature of Ti-V-N steel is attributed to higher content of carbon and Ti, elements that increase hardenability.The lower yield stress for Ti-free steel (V-N) compared with titanium -containig steels (Ti-V-N) is attributed to lower carbon content compared with titanium-containing steel...

show abstract

Relevant aspects of allotriomorphic and idiomorphic ferrite transformation kinetics

Cited by 8 publications

References 23 publications

Influence of V Precipitates on Acicular Ferrite Transformation Part 1: The Role of Nitrogen

Influence of V Precipitates on Acicular Ferrite Transformation Part 1: The Role of Nitrogen

Influence of V Precipitates on Acicular Ferrite Transformation Part 2: Transformation Kinetics

Austenite decomposition in medium carbon microalloyed steels: Mechanism, structure and properties

Contact Info

Product

Resources

About