Surprisingly TiN coated samples showed even lower resistances than their bulk samples. Regardless of the coating type the blank unformed samples provided the highest corrosion resistance. Wang, Y. Investigations on a Ni-Cr enriched layer by Feng K. The specimens were fabricated by sputtering Nb, Cr and using N2 as reaction gas. Cha states that concerning the interfacial resistance the chromium amount in the layer does not make much of a difference but the influence in the gas ratio is noticeable.
The corrosion resistance improved significantly compared to the bare material but pitting corrosion appears with certain coating process parameters. In the work done by Zhang et al. The results were displayed in a tafel plot. The corrosion resistance of the coated samples is higher than its uncoated substrate but not as good as the L specimen.
But the treated Al samples exhibits lower contact resistance than the L thus increasing the power of the cell. Yang et al. Their published results showed good corrosion and low interfacial resistances for the prepared samples. They also discovered that the addition of vanadium in the specimen influence the external nitride layer positively. A TEM cross section image of the nitride surface structure can be seen in Fig.
An increase in corrosion resistance was also observed for the cathodic environment. Unfortunately it exhibited poor corrosion resistance properties in the anodic conditions.
No significant effects were discovered for the low temperature PIII compared to the bulk material. Although the high temperature PIII treated specimen showed better results for the corrosion as well as for the interfacial resistance analysis.
Brady et al. They showed that the modified FeCr-4V possessed the best potential for the usage as bipolar plate materials compared to its untreated material and L alloy. In the work done by Hong et al. As filler mostly a metallic or carbon powder is used. The amount of the electrical conductive material needs to be high enough so the distance between the particles undermatches 10 nm to bring the composite above the percolation threshold where the conductivity of the composite increases excursively [63].
Carbon nanotubes CNT as a layer between steel plate and coating Yang-Bok Lee and his colleagues [45] showed, that a CNT layer between a stainless steel plate and a coating of a pressure molded polypropylene-graphite-carbon fiber composite decreases the interfacial contact resistance. The corrosion resistance test results showed that the chosen polymer-composite is a good protection for the stainless steel.
An improvement was reported for the contact angle being an important factor in the water management of a fuel cell. Resin-Composites A resin-composite-coating was tested by Kitta et al. They coated a stainless steel plate with a thin composite film of bisphenol A-type epoxy resin and 60 vol.
Additionally to the coating they brought a rib structure on the plate using a different resin graphite system. In this case the researchers used a cresol-novolak-type epoxy resin with 70 vol. The area specific resistance of the described plate was reported as Azim et al.
They showed that at an amount of 55 vol. Rubber-Titanium nitride-Composite Another approach to reduce the interfacial contact resistance of stainless steel bipolar plates is shown by Kumagai, M. The stainless steel plates were coated with a suspension of TiN and styrene butadiene rubber SBR by using electro-phoretical deposition.
As shown in Fig. This effect is due to the elasticity of the SBR. Qualitative diagram of the cell voltage loss of the treated specimen [40] Intrinsic Intrinsically conductive polymers were first discovered by Letheby in In the Nobel Prize in Chemistry was given to three scientists who have revolutionized the development of electrically conductive polymers.
The conductivity is realized through conjugated double bonds along the backbone of the polymer seen in Fig. A very low electrical resistance of these polymers can be reached by oxidation. Chemical structure of Poly pyrrole PPy left , Polythiophene PTh right [7] Besides the electrical conductivity and light emission of the polymer the corrosion protection properties were also investigated.
The coatings were brought onto the steel surface by galvanostatic deposition for polypyrrole and by cyclic voltametrie deposition for polyaniline. The PPy were placed first as an inner layer, following covered by the PAni. Electrochemical measurements showed a better corrosion resistance in 0.
Though the corrosion current was similar to that of the bulk specimen it was rather an indication of the oxidation reduction reaction than the corrosion of the substrate material. The chemical stability for the composite coating was higher than that of the single layer PPy after 36 days exposure to the electrolyte. Joseph et al. The coated plates showed improved corrosion resistances in fuel cell environment.
SCE were achieved after the third deposition cycle. The contact resistance was higher for the PPy and PAni coated steel samples compared to the bulk substrate. The results show that the coatings are capable of reducing the corrosion current.
Nevertheless the protective properties did not last for longer times of immersion in an electrolyte of 0. Subsequent tests showed the incorporation of polymer particles of DBSA doped PPy into the metal oxide film on the substrate surface. Defect detection for coatings Defects of coatings on metallic bipolar plates can be detected in many different ways.
Invasive analytical methods are e. More elegant results are provided by the non-invasive methods such as localized electrochemical impedance spectroscopy LEIS , the ultrasonic pulse-echo scheme or the scanning kelvin probe. Defects like pinholes as well as high porous coatings are examples for high corrosion currents so they might be detectable directly through an electrochemical corrosion test. Wooh and Wei reported in [85] a high-fidelity ultrasonic pulse-echo scheme for detecting delamination in thin films.
They stated that the influence of surrounding noises can interfere with the results of the analytical method. Localized EIS, another approach of detecting defects in coatings, is displayed in the work by Dong et al. An LEIS diagram around a defect in a coating after one day of immersion in an alkaline test solution consisting of 0. SCE is shown in Fig. LEIS maps around the defect [15]5 7. Summary and conclusion Metals as bipolar plates in energy conversion systems offer great opportunities.
The high corrosion and the interfacial contact resistance hinder their commercialized usage as substrate materials. During the last decades different approaches were reported by researchers worldwide. To this day it is not clear which the optimal treatment for metals is 5This article was published in Electrochemica Acta, 54, Dong, C. Many modifications on metals provide good potentials. Solutions like electro-plating metals or coating them with conductive polymer exhibit similar difficulties by delamination of the coatings.
Those defects are mostly noticed through high corrosion currents and degradation of the cells. Modifications on the metal surface through CVD or PVD sometimes show similar challenges for delamination as polymer coatings but they also provide very high corrosion potentials and low contact resistances. Some researcher groups report that the best results are achieved by combining methods like electro-plating and nitration by Wang [74].
For further improvements in surface treatments and for designing new of coatings for metals, noninvasive defect detection methods are a essential research topic. These methods will be able to provide in-situ results for metallic bipolar plates in e. Amorphous metals are possibly the most interesting approach. These bulk metallic glasses provide a good prospect for the use as bipolar plates in energy conversion systems due to their corrosion resistance and manufacturing method.
But no long term studies have been made so far. In former times treating metal alloys to overcome environmental challenges was little more than a niche market. This evolved into a field of greater importance, so that many researchers develop, advance and combine ways to coat metals or even design new metal alloys that exhibit the necessary properties. Bearbeitete Auflage [7] Bhadra, S. Progress in Polymer Science 34 — [8] Brady, M. International Journal of hydrogen Energy 36 [12] Chung, C. P et al.
Electrochemica Acta 54 [16] Dong, C. Progress in Organic Coatings 67 [17] Dur, E. Journal of Power Sources [19] Feng, K. International Journal of hydrogen energy 34 [22] Fu, Y. International Journal of hydrogen Energy 36 [33] Jayaraj, J et al.
Journal of Power Sources [40] Kumagai, M. Journal of Power Sources [43] Lee, S. Dental Materials 11 — EP 1 A2 Journal of Power Sources [60] Reuter, M.. Teil 12 Carbon Leitlacke [64] Schoenbein, C. Journal of Power Sources [66] Show, Y et al. Jouranl of Power Sources [69] Trappmann, C. Journal of Power Sources [74] Wang, H. Journal of Power Sources [78] Wang, Y. Applied Surface Science — Corrosion Science 51 [83] Wind, J.
Journal of Power Sources [90] Zhang, M. Introduction It is well known that hot-rolled microalloyed steels derive their overall strength from different strengthening mechanisms that simultaneously operate, such as: solid solution strengthening, hardening by the grain size refinement, precipitation strengthening and transformation induced dislocation strengthening [1].
Precipitation of fine carbonitride particles during thermomechanical processing has been used for many years to improve the mechanical properties of the microalloyed steels, where very small amounts usually below 0.
Each one of these basic precipitation modes will lead to its own characteristic particle distribution, and to generally different effects on steel properties [2]. First systematic investigations on microalloyed steels were carried out in the early sixties at the University of Sheffield [3,4], including initial observations of carbonitride particles by transmission electron microscopy TEM.
According to the early literature on niobium steels yield strength contribution of about MNm-2could be obtained in the as rolled condition due to the presence of fine carbonitride particles, which were observable in the TEM [5]. In principle, these experimental results appeared to be in good agreement with theoretical predictions, based upon the Orowan-Ashby model of precipitation strengthening with carbonitride particles of about 3 nm in diameter [7,8]. The most of the early results cited above were obtained by the observation in TEM of the carbonitride particles but did not determine the origin of the observed carbonitrides [9].
Today, most authors agree that a significant strengthening effect can only be obtained when carbonitride particles precipitate semicoherently in the ferrite phase [12], and that such precipitation will be particularly effective in the case of hot strip products where a combination of shorter rolling times, higher finishing temperatures, and rapid cooling rates after rolling should cause a larger amount of microalloying elements to remain in solution before coiling [13].
Then, a larger volume fraction of very fine particles will thus be available for a more efficient precipitation strengthening during final cooling of the coil. However, no ferrite-nucleated carbonitride particles were found in commercial Nb, NbTi and NbTiV microalloyed steels processed under industrial conditions on a hot strip mill [9, ]. The contribution of dislocation strengthening has been usually neglected in these hot strip steels because of their polygonal ferrite microstructure. It should be realized that many of the results which are presented in the literature have been derived from laboratory tests and processing.
The characteristic hot strip processing conditions during finish rolling high strain rates and short interpass times , however, are difficult to simulate in the laboratory [16]. It is therefore important to study the effects of hot strip rolling in industrially processed materials in order to verify whether real results conform to generally accepted expectations. Origin of carbonitrides and strengthening mechanisms in commercial hot strip microalloyed steels 2.
In austenite, at roughing temperature, the carbonitrides nucleate preferentially on the grain boundaries where simultaneously are occurring the recrystallization processes. Figure 1a shows the extensive precipitation in austenite during the finish rolling TEM dark field image. The diffraction pattern in this Figure indicates the position of the objective aperture which was used for the dark field illumination of carbonitrides. As can be appreciated, the position of the carbonitride reflection showed by the aperture objective position not obeys the Baker- Nutting orientation relationship with respect to the surrounding ferrite, Figure 1b.
No carbonitrides were found that could have formed from supersaturated ferrite after the phase transformation. On the other hand, clear evidence for the presence of interphase precipitation in the form of row formation obeying only one variant of the Baker-Nutting orientation relationship was detected on a coarse scale not very different from the precipitation in austenite in only two grains of the twenty grains carefully observed at TEM, Figure 2.
Interphase precipitation has been associated previously with a very high strengthening potential [17,18], generating yield strengths of more than MPa in a high- titanium steels after isothermal transformations [17].
Interphase precipitation in Nb microalloyed steel[15]. In this sense, when very fine carbonitrides precipitate semicoherently in ferrite, a higher yield strength in the as rolled and coiled product is manifested, while particle coarsening and loss of particle coherence should lead to a lower yield strength after normalising.
Test results indicated yield strength of MPa and MPa before and after the normalising treatment respectively, confirming the absence of fine scale carbonitride precipitation in ferrite during the final cooling and coiling. The weight percent of free nitrogen that remain in solution was calculated for this microalloyed steel. It resulted to be: 0. Thus, the additional contributions from dislocations and precipitation strengthening can conveniently be estimated by subtracting the base value from the total yield strength as determined by tensile testing.
The results are shown in Table 1 for this Nb microalloyed steel. Comparison between yield strength predictions from equation 1 and the results of tensile testing [14]. The results showed in Table 1 give an additional strengthening contribution of about 60 MPa. According to Gladman et al. This value seems to agree with the above difference between the measured and calculated yield strength. Quantitative estimates from several grains where the foil thickness has been measured by counting the number of grain boundary fringes under two-beam contrast conditions indicated an average dislocation density of about cm-2 for this Nb microalloyed steel.
As the interphase precipitation has only occurred in a very small fraction of the grains two of the twenty and in a coarse scale, their influence on the yield strength is also negligible [14]. A Nb steel, above referred which only reached MPa and maintained that strength after normalizing, was used as a reference material.
Chemical compositions and industrial processing conditions of this NbTi steel are shown in Tables 2 and 3. Optical and electron microscopy were used to study the microstructure, and yield strength values before and after normalizing were determined as the average of five tensile tests. Quantitative metallography, yield strength measurements, and structure- property relationships were used for a quantitative estimate of different strengthening contributions [15].
To begin with, the well-known empirical equation 1 served to calculate the contributions from chemical composition and ferrite grain size. Yield strength predictions from Eq. The difference between calculated and measured strength is usually attributed to some additional strengthening mechanism such as carbonitride precipitation or substructure strengthening. The important point in Table 4 is the very large additional strengthening of MPa in the case of the NbTi steel, which was reduced to 69 MPa after normalizing [15].
As mentioned previously, normalizing did not reduce the yield strength of the Nb steel, and the additional strengthening contribution in this steel remained at around 60 MPa, a level very close to the 69 MPa exhibited by the NbTi steel after normalizing. Comparison between yield strength predictions from equation 1 and the results of tensile testing, [15]. Fine carbonitride precipitation was identified in all of the observed grains twenty at TEM, but orientation relationships determined from electron diffraction showed that these particles had nucleated in austenite [15].
In addition, carbonitride distributions appeared to be very similar to the distributions observed in a previous investigated steel [9]. In that case, quantitative metallography and the application of the Orowan—Ashby model of precipitation strengthening had indicated a strengthening contribution of about 60 to 80 MPa for carbonitride particles formed in austenite, in good agreement with the additional strengthening shown in Table 1 for the Nb steel and also for the NbTi steel after normalizing, as it is shown in Table 4.
As in the previous investigations,[9,14] no carbonitrides were found that could have formed from supersaturated ferrite after the phase transformation. On the other hand, clear evidence for the presence of interphase precipitation in the form of row formation [15] was detected in this steel Figure 3. It is apparent; from a comparison between Figures 2 and 3 that interphase precipitation occurred on a much finer scale and thus should have contributed to strength in the case of the NbTi steel.
However, interphase precipitation seemed to occur not very frequently because it was also encountered in only two grains in this steel. On the other hand, the visibility of TEM diffraction contrast from very fine carbonitride particles may require closely controlled sample orientations, which may not have been established in all the ferrite grains under observation for this NbTi microalloyed steel [15].
Interphase precipitation in microalloyed steel[15]. As a first approximation, if it is assumed that about one-half of the total microalloy addition would be available for fine-scale carbonitride precipitation during thermo-mechanical processing [9,15] and applying the Orowan—Ashby model, a maximum strengthening contribution of MPa could be predicted for the interphase precipitation of the NbTi steel shown in Figure 3, based upon a particle size of 2.
If interphase precipitation had occurred in only 25 pct of the ferrite grains, a simple rule of mixtures would thus suggest a strengthening contribution of about 50 MPa in the case of the NbTi steel [15]. Another strengthening contribution could come from the presence of dislocations. In fact, dislocation densities were always higher in the NbTi steel, which again can be explained by its lower transformation temperature in comparison with the Nb steel.
Quantitative estimates from several grains where the foil thickness had been measured by counting the number of grain boundary fringes under two-beam contrast conditions indicated an average dislocation density of 5x cm-2 for the NbTi steel [15]. Such numbers are in reasonable agreement with previous measurements of dislocation densities in microalloyed steels [24] and confirm the possibility of a transformation-induced dislocation substructure even in the case of polygonal ferrite grains.
According to Eq. During normalising, coarsening of fine interphase precipitate distributions and elimination of the dislocation substructure which would not form again during air cooling due to a higher transformation temperature can be expected to reduce the strengthening level to the contribution of austenite precipitation alone 69 MPa according to Table 4.
This later strengthening contribution should survive the effect of normalizing because the formation of carbonitride particles during finish rolling occurs within the range of typical normalizing temperatures [15]. In the case of the previous Nb steel, a higher transformation temperature before coiling would leave the carbonitride precipitation in austenite as the only strengthening mechanism before and after normalizing.
They were compared with the above two microalloyed steels [16]. In contrast, carbon and manganese contents ranged from 0. Chemical compositions are shown in Table 5, together with maximum total volume fractions Vf max for carbonitride precipitation which were calculated assuming appropriated lattice parameters of 0.
Thermomechanical processing conditions are given in Table 6, confirming rather similar processing parameters for all the steels, with the exception of steel NbTiV which was rolled to smaller thickness of 3 mm. The last column in Table 6 shows the yield strength after coiling.
Thermomechanical processing conditions and yield strength after coiling [16]. Such grain refinement can be related to lower transformation temperatures caused by larger carbon and manganese additions to higher strength materials.
Very low dislocations densities were found in the low strength alloy, with quantitative estimates remaining at about cm-2 which would be typical value for well annealed ferrite steel. On the other hand, distinctly higher dislocation densities in the range of to cm-2 were encountered in the high strength steels [16]. Such an increase in dislocation density may also be related to lower transformation temperatures, and a sizeable contribution to yield strength may thus be expected to come from transformation- induced dislocations even in the case of polygonal ferrite microstructures.
According to the Keh equation [15, 25], this contribution could reach about 50 MPa for dislocation densities in the range of 5x cm Carbonitride precipitation in austenite was identified by electron diffraction and was found to be present in all the grains investigated.
Mean particle diameters were observed to increase in proportion with the maximum theoretical precipitate volume fraction [16]. Interphase precipitation was detected in only a small number of grains, but in most of these observations was recognized through row formation. This mode of carbonitride precipitation may have occurred in other grains as well. Preliminary measurements indicated that mean particle diameters of about 2 nm were associated with the smaller sheet spacing, but reached 5 nm in other samples where the sheet spacing were larger.
On one occasion, both larger and smaller sheet spacings were present in the same ferrite grain which probably had transformed during cooling through an extended temperature interval [16]. Quantitative estimates of interphase particles volume fraction gave 3. An Orowan-Ashby analysis showed strengthening contributions of about 60 to MPa from particle volume fractions in austenite in the range of as a function of the particle diameter [16]. Local strengthening contributions from practical interphase precipitation phenomena would reach to MPa.
For a more realistic estimate of the strengthening potential of interphase precipitation in commercial microalloyed steels, it is therefore important to find out more about its heterogeneous particle distributions.
This has been the principal objective of the investigation showed in [26]. In order to investigate the influence on the overall strengthening in hot strip microalloyed steels due to the interphase precipitation, three commercial hot strip steels Nb steel, NbTi-2 and NbTi-3 steels containing different additions of niobium and titanium were selected [26].
Steel selection was based on the following arguments: 1. Different levels of microalloy additions were expected to vary the total amount of carbonitride precipitation.
In particular, real volume fractions and the average particle size of interphase carbonitrides were expected to increase for larger values of Vfmax. Different base compositions, with particular attention to carbon and manganese contents, were supposed to modify the transformation temperature and, as a consequence, to change mean spacings of the interphase precipitation sheets [27].
Thermomechanical processing conditions were desired to be similar, as it is shown in Table 6. Yield strength and additional strengthening contributions from equation 1 in the as coiled and after normalising conditions [26].
Several points should be emphasized: 1. A significant part of the differences in yield strength for the as rolled condition was caused by additional strengthening mechanisms see last column in Table 8. The normalising treatment drastically reduced the additional strengthening contributions in steels NbTi-2 and NbTi-3, but not in steel Nb.
After normalising, additional strengthening contributions were similar for all three steels. The microhardness measurements carried out on individual ferrite grains with the aim of determining the percentage of grains with and without interphase precipitation [26] show some aspects that should be emphasized, Figure 4: Fig.
Vickers microhardness from individual ferrite grains for the as coiled condition and after normalising. Two separate peaks appeared for both NbTi steels in the as coiled condition, with a very distinct peak separation in the case of steel NbTi-3, about of 40 MPa.
Ferrite grains in steel Nb did not show a second hardness peak in the as coiled condition, and their medium hardness values were not affected by normalising. In case of the NbTi steels, normalising removed not only the second higher hardness peak but also reduced the level of the lower hardness peak. As shown in [26], these findings are consistent with different degrees of precipitation and dislocation strengthening in the as rolled condition, and with the effects of particle coarsening and dislocation removal due to normalising.
A detailed TEM investigation was carried out on steel NbTi-3 in order to evaluate the role of precipitation hardening in both as rolled and normalised conditions [26]. No additional carbonitride populations were found that could have formed in supersaturated ferrite, despite the unusually large microalloy addition of 0.
On the other hand, interphase carbonitrides were relatively large and observed frequently, indicating that a substantial fraction of the microalloy addition had remained in solution at the time of transformation. Furthermore, some form of austenite precipitation was encountered in all the ferrite grains that were investigated.
Interphase precipitation was present in only some of the ferrite grains. Occasionally, grains were found to be covered completely by carbonitrides in a row formation. On other occasions, interphase precipitation would occupy only parts of a particular ferrite grain.
The presence of interphase precipitation in only part of a given ferrite grain must therefore be accepted as a real phenomenon [26]. During a TEM study of a large number of grains, of which more than a hundred exhibited interphase precipitation, it was found that random particle distributions were dominant only when thin foils had been prepared parallel to the rolling plane, while row formation was encountered very frequently in longitudinal and transverse sections.
Such observations can only be explained by a preferred alignment of the interphase precipitation sheets parallel to the rolling plane. As a result of this detailed analysis, interphase precipitation was identified in 27 out of a total of 51 ferrite grains that were investigated.
Thus, about one half of the grains in steel NbTi-3 should have been strengthened by interphase precipitation [26]. The first important observation, therefore, was that many carbonitrides continued to decorate typical deformation subgrain structures. It can thus be concluded that the normalising treatment did not have a major effect on carbonitride distributions that had been formed in austenite, although the average particle size was increased.
The second important observation was that row formation could no longer be detected after normalising. This means that extensive particle coarsening must have occurred during normalising, including the transfer of microalloy atoms from dissolving particles in one of the original interphase precipitation sheets to growing particles located in another sheet [26].
Another important result of normalising was the reduction in dislocation density. Quantification of local strengthening contributions in the NbTi-3 steel showed two aspects that should be emphasised in those grains strengthened by both austenite and interphase precipitation, Table 9.
First, the total carbonitride volume fraction after normalising Second, the total level of precipitation strengthening for the as rolled and coiled condition was calculated by using a new average particle size 5. Substructural strengthening contributions in steel NbTi-3 [26]. It can be seen from Table 9, that the largest individual contribution was associated with the interphase precipitation mode, yielding average values of MPa, against 92 MPa for carbonitride precipitation in austenite and 64 MPa for dislocation hardening.
For the overall strength of the steel, it is claimed in [26] that the interphase precipitation should be less effective, because it occurs only in some fraction of the ferrite grains. In addition, moving dislocations do not distinguish between the origin of the carbonitride obstacles that have to be overcome by Orowan bowing. The difference, of only Microhardness measurements can be used in principle to detect additional strengthening mechanisms which operate in only part of the ferrite grains.
New kinetic approaches applied to reactions during tempering in low-alloy steels 3. Extensive studies have been carried out to understand and to model the mechanisms that take place during the tempering of steels. Although it is well accepted that models based on physical principles rather than empirical data fitting give a better understanding of the individual mechanisms which occur on tempering, these models do not contemplate the complexity with which the reactions proceed in each situation T-t.
In this sense, many models do not consider the overlapping of precipitation processes of different chemical natures [29, 30], and when they are taken into account, specific nucleation rates are assessed to fit the entire experimental data without considering the change that the nucleation rate could have during the progress of the reaction [31].
Fitting such models to the entire experimental curve of the fraction transformed could therefore, in certain circumstances result in the prediction of unrealistic kinetic parameters [33]. In other works some attempts have been made [] to deconvolute such experimental master curves into components due to individual processes, but it is difficult to see whether some of these fitting parameters have any physical meaning. Recently, a general modular model for both isothermal and isochronal kinetics of phase transformations in solid state has been published [38, 39].
This model incorporates a choice of nucleation nucleation of mixed nature and growth mechanisms, as well as impingement. Also, the JMAK formulation has been deeply modified to suit isochronal case [], but these analytical approaches need of the nucleation protocols in order to provide a suitable description of phase transformation kinetics during both isothermal and isochronal heat treatments.
In the following, an overview is given about the kinetic theory of overlapping phase transformations KTOPT [43] which is based on the Avrami model. This new approach permits the determination of the kinetic parameters n, k for simultaneous diffusion- controlled precipitation reactions based on the knowledge of a specific macroscopic parameter P t , chosen to study the ongoing reaction.
The present approach does not need to assume nucleation and growth protocols in its formulation to fit the experimental data. This new approach [43] has the particularity of calculating the kinetic parameters in defined work intervals of the fraction transformed curve rather than for the entire curve where the overlapping effect is present.
However, the sample takes a certain time to reach the temperature of the isothermal treatment. During this small time interval, the sample is already undergoing heat treatment, so the beginning of the transformations may be prior to that of temperature stabilization corresponding to the isothermal regime, and therefore; the real initiation of the precipitation processes is unknown. Thus, it is necessary to begin the study not from the origin of the data obtained from the measuring equipment but from the moment of time when the isothermal regime is reached.
Thus, l considering the definition of the fraction transformed, Eq. BOS derives its name … An alloy is a uniform mixture. It is made up of two or more chemical elements, of which at least one is a metal.
An alloy has properties different from the metals it is made of. Among these is mild steel, a commonly used term describing a general type of steel. Large products made of high alloy steels, stainless steels and Ni-based superalloys are said to be difficult to produce due to segregation, transformation property and … 1. Since the influence of carbon on mechanical properties of iron is much larger than other alloying elements.
The atomic diameter of carbon is less than the interstices between iron atoms and the carbon goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the Alloy steel is one of the most versatile steels in the world, with a diverse range of elemental properties and specifications used in a variety of exotic mission-critical applications that demand superior performance under high pressure environments.
Stainless steel alloys, as a rule, contain 10 percent or more chromium. The most popular materials for exhaust valves, however, are austenitic stainless steel alloys such standard steel, a range of 0. General Physical Metallurgy Concepts common to all alloy systems 2. Chemical Bonding, Atom Size, Lattices, Crystals and Crystalline Defects, Solid Solutions, Alloying and Microstructures Our stock includes: stainless steel, alloy steel, galvanized steel, tool steel, aluminum, brass, bronze and copper.
Our hot rolled and cold rolled steel is available in a wide range of shapes including: bars, tubes, sheets and plates. S is a free machining low tensile, low hardenability carbon steel generally supplied in the cold drawn or turned and polished condition, with a typical tensile strength range — Mpa, and Brinell hardness range — Characterised by excellent machinability, moderate weldability, with reasonable strength and ductility.
B2 Low Alloy Steels 1. Classification of steel: According to composition: Steel can be classified according to the composition. The most commercial steel can be classified into two groups 1. The properties and characteristics of this type of steel are according to the carbon content present in it and there is a minor influence on this type of carbon due to the alloying and residual materials.
Plain carbon steel is subdivided into four groups 1. They can be machined and welded nicely and their ductility is greater than high carbon steel Medium: In medium carbon steel the carbon content is from 0.
Due to increased carbon content there is an increase in hardness and tensile strength and decrease in ductility. And its machining and welding is difficult than low carbon steel due to increased content of carbon.
High: In high carbon steel the carbon content is in between 0. And it is the challenge for welding and machining this type of steel. Very high:. In very high carbon steel the carbon content is up to 1. Example: SAE in which 1 indicates plain carbon non modified steel and contains 0. Alloy steel: Alloy steel is a type of steel in which one or more elements other than carbon have been intentionally added, to produce a desired physical property or characteristic.
Common elements that are added to make alloy steel are molybdenum, manganese, nickel, silicon, boron, chromium , boron and vanadium. There are two types of alloy steel 1. By lowering the carbon content to 0. High alloy steel is highly corrosion resistant with high reliability, and is used extensively in petrochemical, pharmaceutical, and nuclear power plants, heat exchangers, centrifugal separators, driers, pipelines, couplings, valves, bolts, salt manufacturing, exhaust gas desulfurizers, and semiconductor cleaning equipment.
Last two digits: Last two digits indicate carbon concentration in 0. According to application: According to application steel can be classified into two types: 1. Stainless steel 2. Stainless steel is more resistant to stains , corrosion ,and rust than ordinary steel.
It is also called a corrosion resistance steel when the alloy type and grade are not detailed, particularly in the aviation industry. Stainless steel is commonly used in table cutlery , jewelry , watch bands , watches , handgun model , pistol , storage tanks , tankers ,food processing plant ,surgical instruments as well as in the aviation industry.
Designation system of alloy steel: AISI has established three-digit system for the stainless steels: 2XX series — chromium-nickel-manganese austenitic stainless steels 3XX series — chromium-nickel austenitic stainless steels 4XX series — chromium martensitic stainless steels or ferritic stainless steels 5XX series — low chromium martensitic stainless steels Tool and die steels: Tool and die steels are high carbon steels either carbon or alloy possessing high hardness, strength and wear resistance.
With carbon content between 0. Tool steels are heat treatable. In order to increase hardness and wear resistance of tool steels, alloying elements forming hard and stable carbides chromium, tungsten, vanadium, manganese, molybdenum are added to the composition.
Tool and die steels are used to shape other metals by cutting, forming , machining and die casting. Tool and die steel is used to make chisels, forging dies, hummers, drills, cutters, shear blades, drills, razors. Tool and die steels can be classified on their use, mechanical properties ,composition and method of heat treatment. Tool steels are made to a number of grades for different applications. For example, SAE refers to mild steel containing 0.
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