Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/416—Systems
  • G01N27/4161—Systems measuring the voltage and using a constant current supply, e.g. chronopotentiometry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/20—Metals
  • G01N33/204—Structure thereof, e.g. crystal structure

Definitions

• the present invention relates to a method for assessing the presence of undesirable phases in steel samples
• the EPR test procedure has been used to detect sigma phase in duplex stainless steels
• the EPR test does not detect harmful phases themselves but rather detects depletion of chromium and/or molybdenum m the base steel around each particle of the phase in question These elements are beneficial for corrosion resistance, so the depleted zones have less corrosion resistance
• EPR tests have been used m the field as well as the laboratory, but suffer from a number of recognised disadvantages such as complex control hardware, use of aggressive acids, need for de-oxygenation of the acid, different procedures and calibrations required for every steel, limited sensitivity, and general slowness
• disadvantages such as complex control hardware, use of aggressive acids, need for de-oxygenation of the acid, different procedures and calibrations required for every steel, limited sensitivity, and general slowness
• There is however a further overriding disadvantage that makes the EPR procedure less approp ⁇ ate for the detection of sigma phase that is although the sigma phase itself has roughly the same composition whether formed at low or high temperature, the associated depleted zone has a radically different composition depending on temperature The higher the temperature, the less the amount of chromium and molybdenum depletion and the more danger that the EPR test will give a false negative result
• a method for assessing the presence of undesirable phases m a steel sample wherein an anodising current is applied to the sample such that the sample is exposed to an anodising potential, and increases in the anodising potential are monitored, a relatively low rate of increase m the anodising potential indicating the presence of undesirable phases
• the sample may be passivated by immersion in an alkali solution Alternatively or in addition the sample may be passivated by applying an anodising current to the sample
• the rate of increase in the anodising potential may be monitored for example by momto ⁇ ng the slope of a curve representing the va ⁇ ation of anodising potential with time, or by measu ⁇ ng the anodising potential a predetermined time after the initiation of the anodising cu ⁇ ent, or by measuring the time taken for the anodising potential to reach a predetermined potential.
• the invention provides an electrochemical procedure and associated electrochemical probe that make it possible to measure the amount of chromium-rich phases in stainless steel either directly or by detecting chromium depleted regions. It has been developed for 25% chromium duplex stainless steels but would work on other materials including less-alloyed duplex steels, weld metals, and high-alloy austenitic steels. Indeed the test has features that make it uniquely flexible when used on different steels.
• Figure 1 illustrates the variation with time of an anodising potential applied in accordance with the method of the present invention
• Figures 2 to 6 illustrate respectively the linear fit of each of five curves shown in Figure 1 ;
• Figure 7 illustrates the reproducibility of data such as that shown in Figure 1 ;
• Figures 8 and 9 show results indicating the potential impact of thermal pre- treatment to variations in the monitored potential
• Figure 10 is a series of micrographs which illustrate the micro structures of specimens tested using the invention.
• Figure 11 illustrates Charpy impact failure as a function of Ferrite meter reading for the specimens
• Figures 12 and 13 illustrate the variation over time of the potential at the specimens when the method is applied with different cu ⁇ ents
• Figure 14 illustrates the potential at the specimens after a particular time for different currents
• Figure 15 illustrates the time taken for the potential at the specimens to reach a particular value for different currents
• Figures 16, 17 and 18 illustrate variations with time of anodising potentials applied to a sample in a 5% NaCl solution
• Figure 19 shows results similar to those of Figure 1 but with a modified pre- passivation treatment
• Figure 20 illustrates va ⁇ ations with time of potentials applied to a se ⁇ es of specimens in a 5% NaCl solution
• Figure 21 illustrates vanations with time of potentials applied to the se ⁇ es of specimens in the 5% NaCl solution at 50C
• Figure 22 illustrates va ⁇ ations of time of potentials applied to the se ⁇ es of specimens in a more concentrated NaCl solution
• Figure 23 illustrates the potential at the senes of specimens after 500s for a range of temperatures
• Figure 24 illustrates va ⁇ ations with time of potentials applied to a se ⁇ es of specimens in a bromide solution
• Figure 25 illustrates the Charpy failure energy as a function of the potential at the specimens after 500s
• Data analysis can be via differentiation of the potential-time relation
• the presence of sigma phase is indicated by a minimum in the differential curve, or in any case a reduced slope
• the amount of sigma phase can be quantified by the potential reached after a given pe ⁇ od of anodising, by the slope over a fixed interval of time, or alternatively some function that includes the duration and depth of the minimum in the differentiated curve
• the five curves shown represent the va ⁇ ation in the anodising potential with time for a stainless steel sample treated for 1000 seconds at the five respective temperatures 650°C, 750°C, 850°C, 950°C and 1050°C These temperatures are shown in Figure 1 with the figures in brackets after the treatment temperatures representing toughness values in joules de ⁇ ved using the Charpy test
• the mate ⁇ al tested contained some mtermetalhcs before heat treatment The results were de ⁇ ved after immersing the samples in a 0 1 M Na 2 C0 3 alkali solution A pre- passivation anodising potential of 250mV was applied for 1 mmute and thereafter an anode cu ⁇ ent of lO ⁇ A/cm" was applied
• Figures 2 to 6 illustrate the linear fit of the five curves shown m Figure 1 It will be seen that for the sample heated to 1050°C, the slope of the curve m the interval 100 seconds to 200 seconds is 1 77 This contrasts with figures for the samples treated at 650, 750, 850 and 950°C of 0 798, 0 0923, 0 158 and 0 561 respectively Thus simply by monito ⁇ ng the slope of the curves in the 100 to 200 second interval a clear distinction can be made as between samples that are substantially free of delete ⁇ ous phases as against samples containing significant amounts such as 1% of delete ⁇ ous phases
• the above example of the invention may be performed using a fibre-tip penlike device containing two electrodes, connected to an analogue to digital card and computer
• the double cu ⁇ ent-step procedure described can operate on the basis of a simple voltage measurement If an initial passivation cu ⁇ ent was not applied, it would be necessary to use a potentiostat to give passivation at controlled potential
• the monitored anodising potential as illustrated m Figure 1 can be calibrated to represent a measure of sigma phase present in the sample The time per measurement may be about 5 minutes but could be reduced to for example 1 or 2 minutes
• Different pre-passivation treatments and cu ⁇ ents can be used to give two different outputs, that is one indicating total mtermetalhc content (relevant to co ⁇ osion as well as embnttlement) or emb ⁇ ttlmg phases only
• test will perform better on real components than on isothermally heated material where the low temperature phases are emphasised. In any case, the test provides a fast way to certify that a material is free from all intermetallic phases. This represents an important limiting case. It is estimated that the sensitivity of this technique is 0.1% for the low temperature phases and 0.5% for sigma.
• a second set of data has been generated using the procedure in accordance with the present invention (also refe ⁇ ed to herein as the carbonate test).
• the test material used to obtain the second set of data was Zeron 100 obtained from Weir Materials (0.017C, 0.24Si, 0.54Mn, 0.021P, 0.001S, 24.97Cr, 3.58Mo, 6.97Ni, 0.52Cu, 0.54W, 0.22N, balance Fe), and was in the form of 16 mm-diameter round bar. Samples of the bar were either used as-received, or heat-treated for 100, 300 or 1000 s in air at 675 or 825C. In each case the heat treatment was followed by water quenching.
• FIG. 10 The microstructures of the specimens are illustrated in micrographs shown in figure 10 (the micrographs comprise cross-sections of the bars with electrolytic KOH etch).
• Figure 10a is an untreated specimen
• figures 10b to lOd are specimens that were heat treated at 675C for 100s, 300s and 1000s respectively.
• Figures lOe to lOg are specimens that were heat treated at 825C for 100s, 300s and 1000s respectively.
• the Charpy impact energy of the specimens was tested at -50C.
• the resulting broken specimens were mounted on end in epoxy resin for electrochemical testing. A 1 cm 2 area of cross-section just behind the fracture plane of each broken specimen was exposed to the test solution. All the electrochemical testing was performed on surfaces polished to a 1 micron finish.
• test condition 825C, 100 s
• three different specimens were tested electrochemically. These are designated A, B and C in the results discussed below.
• For other conditions only one specimen was tested electrochemically.
• Ferrite meter measurements were obtained for each specimen to provide a comparison with the procedure according to the present invention.
• the Ferrite meter readings are plotted against Charpy impact failure energy in figure 11.
• Each point on the graph shown in figure 11 is an average for three specimens that have undergone the same amount of heat treatment.
• Figure 11 demonstrates that the Ferrite meter measurements lack discrimination at lower levels of deleterious phase content (i.e. the right hand end of the graph).
• the carbonate test consisted of applying a passivation potential of 250 mV for 1 minute followed by an anodic cu ⁇ ent in the range 10 to 50 ⁇ A/cm . The potential was monitored for 10 minutes.
• Figure 12 shows an example of raw data obtained using a relatively low cu ⁇ ent of 10 ⁇ A, the potential at each test specimen being shown as a function of time.
• Figure 13 shows the equivalent data for 50 ⁇ A. The sensitivity of the carbonate test can be seen to vary significantly when different cu ⁇ ents are used.
• Figure 14 is graph which plots the Charpy impact failure energy against the potential after 500 s.
• Each line of the graph co ⁇ esponds to a different cu ⁇ ent in the range lO ⁇ A to 50 ⁇ A.
• Each point on the graph represents a different test specimen
• a point nearest the left hand end of the graph represents a specimen which has undergone heat treatment at 825C for 1000s
• the second and third points on the line represent specimens which have undergone heat treatment at 825C for 500s and 300s respectively.
• the fourth point on the line represents a specimen which has undergone heat treatment at 675C for 1000s
• the fifth and sixth points on the line represent specimens which have undergone heat treatment at 675C for 500s and 300s respectively.
• the final point on the line represents an untreated specimen.
• Figure 14 shows that the carbonate test gives a strong co ⁇ elation between the Charpy impact failure energy and the potential after 500s. Furthermore, a significant variation of the form of the co ⁇ elation may be obtained by using a different cu ⁇ ent.
• the cu ⁇ ent required for a given carbonate test may be selected on the basis of a required sensitivity. It can be seen from figure 14 that an appropriate cu ⁇ ent may be chosen (for example 15 ⁇ A or lO ⁇ A) such that the carbonate test provides a "threshold" type of evaluation where the potential response drops suddenly at a particular Charpy impact failure value.
• the results of the carbonate test may also be analysed using a graph which shows Charpy impact failure energy against the time taken to reach a particular potential (750mV), as shown in figure 15.
• 750mV Charpy impact failure energy against the time taken to reach a particular potential
• each line in figure 15 represents a different current, and each point represents a different specimen.
• Figure 15 shows a strong co ⁇ elation between the time taken to reach a potential of 750m V and the Charpy impact failure energy.
• the correlation is close to being linear, demonstrating that interpretation of the data in this way may provide a substantially linear electrochemical measurement of the Charpy impact failure energy of a specimen.
• a significant variation of the form of the co ⁇ elation may be obtained by selection of a different cu ⁇ ent.
• the cu ⁇ ent required for a given carbonate test may be selected on the basis of a required sensitivity.
• Figures 17 and 18 show similar data for 50 and 10 ⁇ A/cm 2 respectively. Potentials shown are on the SCE scale; a sustained potential around +1V indicates uniform transpassive dissolution, not pit-like localised co ⁇ osion. The following features are of note:
• the steel will not show pitting co ⁇ osion at ambient temperature if there is no chromium depletion, so if there is no sigma phase the potential rises up to the applicable transpassivity potential and stays constant. However the chromium depleted zones do show pit-like co ⁇ osion. Thus in summary, if chromium depletion is present, the potential either reaches a constant value far below the transpassivity potential (for a severe case where the chromium-depleted zones are connected throughout the microstructure) or shows an a ⁇ est at an intermediate potential before rising to the transpassivity potential (for a case where there are finite chromium- depleted zones connected to the steel surface).
• the present invention was used to indicate the presence of chromium depleted regions in a further set of specimens (the test is refe ⁇ ed to hereafter as the chloride test).
• the chloride test was carried out using the specimens that were used to obtain the second set of data (i.e. the Zeron 100 specimens described above).
• Figure 20 shows raw data for room temperature chloride testing of the specimens. Only the specimen that has been heat treated at 825C for 1000s shows a persistent low potential indicative of localised co ⁇ osion due to solute depletion zones in the alloy.
• Figure 23 shows the co ⁇ elation between the Charpy impact failure energy and the potential after 500s for specimens tested at a range of different temperatures.
• Each line on the graph represents a different testing temperature.
• Each point on the graph represents a sample that has been subjected to a specific heat treatment (the a ⁇ angement of the points is the same as that described above in relation to figure 14).
• the co ⁇ elation between Charpy impact failure energy and the potential after 500s becomes increasingly linear as the temperature is increased.
• At 50C a substantially linear co ⁇ elation with impact properties is obtained. This co ⁇ elation could be tuned still further if required; the important point is that moderate degrees of deleterious phase precipitation are quantifiable and can be separated from the most severe conditions.
• Sodium bromide may be used in place of, or in combination with, sodium chloride.
• the test was carried out with a 0.85 molar sodium bromide solution using the Zeron 100 specimens described above. The test was carried out at room temperature, using a current of 10 ⁇ A cm 2 .
• Figure 24 shows the variation of potential for each Zeron 100 specimen from 0 to 500 seconds. Good discrimination between the samples is provided by the test.
• the potential of the control sample increases smoothly over time and tends towards a stable value of around IV.
• the potential of the heat treated samples initially increased at the same rate as the control sample, but then increased at a lower rate and generally stabilised at a range of values significantly below IV.
• the rate of increase of potential and the final potential are seen to have a close dependency upon the heat treatment applied to the samples.
• Figure 25 shows the potential at 500s for each sample, plotted against the Charpy failure energies of the samples. It can be seen from figure 25 that there is a strong co ⁇ elation between the potential at 500s and the Charpy failure energy, the co ⁇ elation being almost linear except for very low or very high Charpy failure energies.
• the sodium bromide test is particularly advantageous because it provides good discrimination between samples at room temperature (a similar level of discrimination to that provided by the sodium chloride test at 50C).
• the bromide solution does more than just reproduce the chloride result at a lower temperature.
• the chloride test is very sensitive to molybdenum, whereas the bromide test detects chromium depletion (molybdenum is dissolved much more easily in the presence of bromide ions). This means that there will be instances in which both tests may be used to provide complementary information.
• One example where both test are advantageous is where a particular deleterious phase (chi phase) has higher molybdenum level than another (sigma), so that the region around each chi particle is more depleted in molybdenum than the region around each sigma particle.
• chloride test responds differently to sigma and chi because chloride is very sensitive to molybdenum, while the bromide test will just detect the chromium depletion which is about the same for sigma and chi, providing complementary information. If sigma and chi are equally deleterious to mechanical properties, then the bromide test will give a better relationship with mechanical properties.
• the chloride and bromide solutions may be mixed together to provide a single test. This is advantageous because the bromide will tend to eliminate the beneficial effect of alloyed molybdenum on co ⁇ osion. Consequently, a mixed chloride and bromide solution test may be carried out at room temperature.
• the mixed solution test can be tuned to provide intermediate discrimination, and emphasise particular levels of sigmatisation.
• the mixing could be tuned to emphasise the relative effects of chi phase (which causes heavy molybdenum depletion) and sigma phase (which does not cause significant molybdenum depletion) by selecting the relative proportions of chloride and bromide.

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