Austenites

Austenitic steels retain the ccp structure right down to room temperature. For this reason these steels cannot be hardened by quenching. 

Determining Residual Austenite with the Eddy Current Method. 

While quenched steel with carbon above 0,6%, the temperature of the end martensite transformation is below zero, thus the transformation of austenite into martensite is incomplete and this remaining cooled austenite is called residual austenite. 

Residual austenite is a steel structure which during cooling at martensite transformation temperature is not completely converted into martensite and remains unchanged at room temperature together with martensite. 

The structure of residual austenite is metastable, during exploitation it may panially transform into bainite, whereas during quenching this transformation may be caused by the freezing out processing. The transformation of residual austenite into bainite is connected with volume change, whereas diminishing the content of austenite in martensite by 1% causes a 0,07% increase of its volume. 

The eddy current method is used for the quantitative evaluation of residual austenite contents in the martensite struaure. 

The principle physical phenomenon of applying the eddy current method for evaluating the amount of residual austenite in the structure of quenched steel is magnetic induction, involving the influence of the changeable magnetic field on the studied area, found under the probe. 

Structure defects decrease conductivity of the studied material, and then the intensity of the induced magnetic field is small and the signal received by the probe Hp is big (Fig.2). Low conductivity of austenite is a defects of the structure in case of residual austenite in the martensite structure, which with regard to the magnesite structure is as 1 5. Eddy currents produced in the studied area are subject to excitation in effect of small conductivity of austenite grains in the structure of the studied material. 

Eddy current devices for detecting residual austenite comprise ... 

The detection of residual austenite in fact requires average frequency, however for comparison reasons (reference) with a different recognized method, it is recommended to use high frequency, as with high frequency of eddy currents the penetration depth is comparable in the diffraction method and eddy current method. 

Attention should be given in the fact, that penetration of eddy currents in residual austenite will be slightly deeper than in the martensite structure of steel, as austenite shows low electrical conductivity. The signal originatimg from the austenite structure will be amplified in effect of the influence of the structure found at greater depth. There will be no error as the method of measurement is compartable and the samples made for reference purposes will have the same structure as the studied part. 

For detecting and percentage evaluation of the participation of the amount of austenite in the quenched structure of hyper-eutectoidal steel, devices manufactured by CMP type WIROTEST 202 and WIROTEST 12 finish (Table 1.) are applied. These devices allow to detea and evaluate the content of residual austenite as well as form the signal for part segregation with austenite content above the allowed amount, as well as parts with grinding burning... 

Calibration of devices for residual austenite control in elements after thermal processing... 

The determining of sorting limits of steel parts after thermal processing in order to eliminate these, which indicate exceeded allowed content of residual austenite, requires elements of identical shape and dimensions, as the studied parts, and with known content of residual austenite. Such elements serve to define the sorting thresold, during manual control as well as automatic... 

It is known, the residual austenite is not a stable structure and after some time is transformed into a bainite structure, so elements used for calibrating sorting thresholds will be unstable, and thus unrealiable Thus special reference samples showing structure stability should be used. 

Fig. 3. A diagram of the sample with residual austenite tor calibrating eddy current devices...

Thus they have been replaced with elements with a martensite structure mixture fully transformed by the zero processing and mounted in openings with insens of austenite steel (Fig.3). Thus prepared elements for caUbration. will be stable with time and will not cause any indication changes during exploitation. 

The percentage share of the inserts made austenite steel in the martensite structure is refered to the visual field of the probe in the given location. Every probe after performance is given a characteristics, in which the visual field is given, determined using special devices defining the visual field at different distances from the tested object. 

Results of tests of the occurrence of residual austenite in bearing steel... 

Fig 4 Graph of the dependence of Wirotest 202 indications as function of the residual austenite content... 

Segregation of bearings, with regard to residual austenite was performed with the aid of WIROTEST 202 and WIROTEST 12 finish. Selected rings with defined indications were subject to metalographic tests, in order to state whether residual austenite occurs, and then using the diffraction method, the percentage content of residual austenite. 

The showm in Fig. 4 graph of the dependence of the WIROTEST 202 as a function of the residual austenite content, allows to evaluate the content of residual austenite in steel in the scope for 5 +/-100%. 

It should be emphasised, that further tests will allow to obtain a higher precision of detectability of residual austenite, especially within the scope below 5%. 

The elaborated method of applying elements for calibration of devices composed of the body with a nartensite structure, with elements made of austenite steel placed in it, allows to ensure calibration repeatability,... 

Suwalski L., Kucharski Z., Lukasik M., Luty W. Utilised of Mossbauer spectroscopy for measuring residual austenite in bearing steel MOC IMP No 67, 1985,... 

Wyszkowska J. Kinetics of growth of the austenite grain Issued by IMP 1971,... 

Lampe J. Preparing surfaces for radiological determining of residual austenite MOC IMP No 47,1980. 

The ultrasonic testing of anisotropic austenitic steel welds is a commonly used method in nondestructive testing. Nevertheless, it is often a problem to analyze the received signals in a satisfactory way. Computer simulation of ultrasonics has turned out to be a very helpful tool to gather information and to improve the physical understanding of complicated wave phenomena inside the samples. 

In this paper, 2D simulations of the ultrasonic testing of different types of (transversely isotropic) austenitic V-butt welds are presented they have been obtained with an EFIT-code developed by Marklein [5]. 

The austenitic and, hence, anisotropic V-butt weld is embedded in isotropic steel it has a width of 10 mm at its baseline and a height of 30 mm. If a notch is present, it has a width of 1 mm and a height of 15 mm it is located at the right-hand side of the V-butt weld. The simulated transducer is a commercial 45°-shear wave probe (MWB45-2E). The parameters varied during the simulations are ... 

Figure 5 Slowness and group velocity diagrams for austenitic weld material perpendicular grain orientation...

Insurance in pressure boundary integrity of NPP unit is strongly influenced by technical capabilities and efficiency of metal examination system. Ordinary ultrasonic examination tools and procedures have limitations in flaw sizing and positioning. The problems arise for welds and repair zones of welds made by filler materials of austenitic type. 

AUGUR information on inspected zones as applied to pearlitic and austenitic weld materials reduces level of ISI results uncertainty and therefore gives additional insurance of pressure boundary integrity. 

Modelling of Ultrasonic Crack Detection in Austenitic Welds. 

The radiographs were made from austenitic (titanium- or niobium-stabilised nickel chromium steels) welds that were double-wall penetrateted by x-rays of 130kV. The wall thiekness is in the range of 8 to 12 mm. 

As described above 2314 ROIs were cut out of several radiographs of austenitic welds and the accessory feamres were calculated. The true data for each radiograph is known by destructive testing and evaluation. The ROIs represent the following dataset ... 

Another example shows a 4 MHz longitudinal wave probe WSY70-4 normally used for testing of coarse grained austenitic material. In this application a high pulse amplitude is... 

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