Detection of Crack and Spallation of Oxide Scales using Acoustic Emission Technique

Acoustic Emission has been finding increasing application for monitoring of aging infrastructures. The materials undergo degradation mechanism during long service exposure at high temperature. Experiments have been carried out on recently developed steel grades P91, P92 and E911 used in thermal power plant materials using acoustic emission (AE) technique under isothermal oxidation in air at 950°C and 1000°C upto 5h. AE parameters count and voltage level shows negligible during isothermal heating for the alloys at these temperatures until the test duration, however a sudden increase is AE activity is found while cooling. An enormous increase in AE activity occurs after the start of cooling has been related to spallation of oxide layers. A very large increase in AE parameters for P92 and E911 alloy at 1000°C during cooling from this temperature has been related to cracking of oxide scale. The oxidation specimen was analyzed using various surface analytical techniques such as scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX).

amplification is done after the sensors detect the activity in order to transmit current long distances without significant loss.In an earlier worker 3 have successfully used this technique to detect the breakaway oxidation of oxide layers formed on 2.25Cr -Mo alloy by continuously heating the specimen from ambient to the set temperature.In the present work to detect the crack and spallation of oxide layers of recently developed P92 and E911 alloys have been carried out for monitoring AE and compared to the conventional P9 and P91 alloys.

MATERIALS Experimental Techniques
Four different low alloy steels namely P9, P91, P92 and E911 were obtained from Indira Gandhi Centre for Atomic Research, Kalpakkam and Forschungszentrum Julich, Germany.The chemical compositions of alloys analyzed using inductively coupled plasma and atomic emission spectroscopy (ICP -AES) technique are shown in table 1.
The received materials were cut into the rectangular specimens of size 10x10x2 mm and the samples were polished up to 800 grit using silicon carbide papers, washed in distilled water, cleaned in acetone and air dried before oxidation test.The specimen was spot welded to a platinum wire waveguide (1mm diameter) at one end and to a copper plate at the other end of the wire.A piezoelectric transducer was placed the welded region of the copper plate to capture the AE being transmitted through the platinum wire waveguide.Acoustic emission tests were carried out in a horizontal furnace fitted with a (32mm) diameter Quartz glass tube.The sample is inserted into the hot zone of horizontal muffle furnace when the desired temperature was attained.The furnace was equipped with a PID (Proportional Integral and Derivative) controller to maintain the uniform and desired temperature.AE monitoring was carried out using a Model 204B system supplied by M/S.Acoustic Emission Technology U.S.A.A band pass filter was used along with 60dB fixed gain amplifier.For all the experiments, a total system gain of 100dB and a threshold voltage of 0.68 volt were used.These setting values were estimated after several trial experiments with the objective of maintaining the background noise to an acceptable minimum.The acoustic signal was recorded using a two-pen volt strip chart recorder.An experimental setup is shown in Fig 1 .The experiments were carried out in two stages; the 5h isothermal heating for 950°C and 3h heating for 1000°C, after that the furnace electrical power was switched off

RESULTS
The Fig 2-5 bottom plot shows voltage level vs time and top plot shows AE events generated during cooling and inside furnace temperature.The acoustic parameters did not show any activity during the heating for specimen oxidized upto 950 and 1000°C.After temperature peaked and cooling begun and large AE activity has obtained due the stress was sufficiently high.Stress generation leads to increasing elastic energy which is stored in the oxide metal composite.If the oxide layer is thin compared to the thickness of the metallic substrate, the elastic energy is stored mainly in the scale.Exceeding the stress limits locally leads to spontaneous transition from a state of high potential energy to a state of low potential energy.This can happen by the formation of cracks through or within the oxide layer or by partial spallation of the scale.As this process occurs, part of the released energy is emitted as propagating  Surface morphology analysis Fig. 6 shows SEM morphology of the oxide scale formed on the alloys at 1000°C in air.A thick and rosette appearance oxide formed for P9 and P91 alloys (Fig 6 (a-b)).However oxide formed P92 and E911 alloys having a rosette appearance with cracks (Fig 6 (c-d)).Cracks were seen during cooling, which deteriorate the scale adherence and increase the oxidation rate.On the other hand, the growth and thermal stresses generated during oxidation may be accommodated by scale cracking, scale spalling from the alloy surface, and the plastic deformation of the matrix 4 .

Analysis of Cross Section
Cross sections of selected oxidized specimens analyzed using SEM/EDAX are shown in Fig [7].The oxide layer seems to be double layer and adhered to the matrix for P9 and P91 alloys.P92 alloy having double oxide layers with spallation in the outer oxide layer.However, E911alloy having double layers with cracks and spallation as shown in 7 (d).The cracks are likely to be related to stresses originating from differences in thermal expansion coefficient between alloy and oxide at high temperatures 5 resulting in tensile stresses in the oxide during cooling.

DISCUSSION
The 9% chromium steels developed during the last three decades are of great interest for components such as water wall, superheater, reheater and steam pipe for power plant application.9Cr -1Mo (P9) and modified 9 -1Mo (P91) alloys cannot be used at higher temperatures and pressure, because of poor creep strength above 593°C.Supercritical power plants generally operate at temperature more than 600°C.For that advanced materials are needed to withstand high temperature and pressure in term of strength, creep and oxidation resistance.To improve the mechanical properties of materials, a small amount of alloying element such as tungsten (W), niobium (Nb) and vanadium (V) are added to conventional 9Cr -1Mo alloy.However, because of high concentration of ferrite stabilizing elements such as W, Nb and Mo, there are chances of deleterious phase formations such as σ or Laves phase.V and Nb combine with C and or N to produce carbides, nitrides or carbonitrides, which form coherent precipitates and help in precipitation strengthening.Modified alloys with these P92 and E911,these having increased the mechanical properties of alloy are shown in Table 2 The P92 and E911 alloys with the addition of active elements such as W and Nb, which improve the mechanical strength and creep strength of these alloys but the oxidation resistance of these alloys as the following order P92<E911<P91<P9 [6].For the comparison of various AE parameters at 1000°C shown in table 3. It can be seen from data, there is an increases in AE parameters for E911 alloy compared with other three because higher amount of molybdenum present in this alloy.There is a general tendency for alloy contains a higher amount of molybdenum to produce more acoustic emission activity 7 than other alloys containing low molybdenum levels.Since as a result of thermodynamic calculations molybdenum can form highly volatile hydroxyl species under these conditions it can be suspected that these increase the tendency to scale cracking and may even have an impeding effect on scale healing.

CONCLUSIONS
Acoustic emission test was carried out over the temperature range of 950° C and 1000°C.The following results were obtained.1.
The lower the oxidation resistance of alloy (E911) is the more AE activity expected 2.
Oxide spalling is responsible for a considerable amount of acoustic activity during cooling 3.
Among the four alloy steels, the acoustic emission was observed in the following order as E911< P92<P91<P9 at 1000°C