Magnetic properties of the plastically deformed AISI 304 SS and the weldments of the same steel were studied to find the feasibility of using magnetic NDT techniques for evaluation of strain induced martensite transformation or the delta ferrites in weldments. Magnetic Hysteresis Loop and Magnetic Barkhausen Emissions were used for evaluation of properties. However these two techniques were found suitable when the martensite amount was more than 18 vol%. A sensor MAGNASENSE was developed which successfully determined the presence of about 6 vol% of martensite and 0.4FN delta ferrite in weldments.
Key Words: Martensite, Delta Ferrite, Magnetic NDT, 304 SS, Magnetic Sensor
Austeritic stainless steel are used mainly for structural application where corrosion related phenomena are not desirable. However, in some stainless steel like AISI 304 SS, the unstable non-magnetic austenitc phase (g
) transformed to a magnetic phase martensite (a
/ ) when plastic deformation takes place . Presence of martensite is stainless steel increases the work hardening capability and thus reduces the formability of 304 SS . It also increases the tendency for stress corrosion crocking and hydrogen embrittlement. There is another way that the austenitic stainless steel can be transformed to a magnetic phase, delta ferrite, when the material undergoes welding process. Delta ferrite is often a desirable constituent in an austeruitic stainless welds  . It prevents hot cracking in the weldment . However, a critical percentage of ferrite is recommended which is about 3-8 vol% depending on the application. Higher this amount would mean the possibility of conversion to brittle phases such as sigma when the material is in service at high temperature. Another situation where it become necessary to restrict the presence of ferrite in welds is when the components or structure has to be non-magnetic. So there is a need to develop non-destructive testing technique by which the magnetic phases that are produced in non-magnetic stainless steel during service. In this study attempts have been made to use various magnetic techniques for evaluation of magnetic phase in plastically deformed and welded AISI-304 Stainless steel.
AISI 304 Stainless steel ( C = 0.07, Mn = 1.95, Cr = 18.7, Ni = 9.0 and Fe = balance) was plastically deformed at various percentage by cold rolling . During the process of cold working, martensite phase was formed and its amount was depend on the percentage of cold working. Four controlled weldment samples of AISI 304 stainless steel with different ferrite contain were also used for the present study.
Optical micrographs were taken for various cold rolled and welded specimen. It might be mentioned that welded specimen were chemically etched using boiling Murakami's reagent for 5 minutes for sample preparation for microstructural study. The composition of the reagent is: 10 gm K3Fe(CN)6, 10 gm KOH, 100 ml distilled water. X-ray diffractograms were taken to measure the amount of martensite phase in different cold worked specimen. Magnetic Hysteresis Loop (MHL) was determined using a surface probe at 50 mHz magnetising frequency. Magnetic Barkhausen Emision (MBE) signal was measured using surface probe ( American Stress Technology, m
Scan 500-1 ) at a magnetising frequency of 120 Hz. A magnetic sensor MAGNASENSE  was developed in the laboratory which was also used to measure in presence of magnetic phase in the AISI 304 stainless steel.
Results & Discussion:
(a) Evaluation of martensite in cold worked AISI 304 SS
The amount of martensite produced due to rolling was measured by X-ray diffractometer and the result is shown in fig-1.
Hysteresis loop with finite coercivity value was observed in the cold rolled sample having 18 vol% of martensite or more.
The typical hysteresis loop for 18 vol% and 39 vol% of martensite are shown in fig-2(a) and fig.-2(b) respectively.
The variation of coericivity with the percentage of martensite presence in cold workd 304SS is shown fig-3. Coercivity increased with the amount of martensite and got saturated at higher volume fraction of martensite. Similar behaviour was also observed by others . Fig-4 shows the magnetising field dependence of Barkhausen voltage for different volume fraction of martensite.
Fig 1: Amount of martensite formed due to cold rolling as determined by XRD
Fig 2: Magnetic hysteresis loops for 18 Vol% (a) and 39 Vol % (b) of martensite phase in cold worked AISI 304 stainless steel|
Fig 3: Change of coercivity with the % of martensite
Fig 4: Magnetisation field dependence of Barkhausen emissions voltage for samples of different volume fraction of martensite.|
MBE signal was observed when martensite volume fraction was about 18%. Hence both the available magnetic NDT techniques are not suitable to monitor martensite in 304 SS when it is below 18 vol%. Hence a new NDT technique is very much needed to evaluate the low volume fraction of martensite. With this view, the sensor MAGNASENSE was developed and the sensor output voltage with respect to the martensite volume fraction is shown in the fig.-5. The sensor was found to detect sensibly about 6 vol% of martensite. A linear variation of the output voltage up to about 20 vol% with a slope 17 mV per vol% of martensite was found. The inset of fig.-5 shows the sensor output voltage within the measured range of martensite.
Fig 5: Variation of sensor output voltage with the % of martensite. The inset shows the variation within the full range of measurement
Fig 6: Microstructure of weldments having delta ferrite numbers 0.4FN(a), 1.82FN (b), 2.3FN (c), 3.65FN (d)
When the percentage of martensite was very low, the regions of magnetic phases were separated in such a distance that neighbouring regions were not ferromagnetically coupled resulting in super paramagnetic nature of the sample and hence the hystresis loop
and Barkhausen signal were not observed below 18 vol% of martensite. However, as the magnetic moment of the super paramagnetic state is higher MAGNASENSE was able to detect the signal. As soon as the volume percentage of martensite increased, the martensite phase regions came closer and were ferromagnetically coupled resulting in appearance of hysteresis loop and Barkhausen emissions signal.
(b) Evaluation of delta ferrite in weldments of AISI 304 SS
Fig.-6 shows the microstructure of the weldments having different percentage of delta ferrites. Delta ferrites are often expressed as Ferrite Number (FN) to have parity of measurement from place to place . At low FN (below 5), volume percentage of ferrite is almost equal to the ferrite number. The output voltage of the sensor, MAGNASENSE, with the FN is shown in the fig.-7. A linear increase of the sensor output voltage with the FN was observed and the results indicated that the sensor could detect down to 0.4FN in weldments.
Fig 7: Variation of sensor output voltage with ferrite number (FN) in weldments
Magnetic non-destructive techniques like Magnetic Hysteresis Loop and Magnetic Barkhausen Emissions, were used for evaluation of martensite in plastically deformed AISI 304 SS. These two techniques were found suitable when the martensite volume fraction was more than 18vol %. A portable magnetic sensor MAGNASENSE was developed which was suitable to detect martensite down to 6 vol% of martensite in plastically deformed AISI 304 SS. This sensor was also found to be suitable to detect delta ferrite down to 0.4FN in weldments.
The authors are grateful to Director of the laboratory for giving permission in publishing the paper. The work is a part of the project sponsored by Dept. Sci. & Tech., Govt. of India.
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