TEM Image Analysis of Fracture Surfaces of P91 Steel and Comparison with Mechanical Properties

In order to improve the efficiency of steam boilers in modern power plants, engineers have to use contemporary materials that enable exploitation at higher parameters, primarily operating temperature and pressure, with resistance to different forms of corrosion at the same time. For that purpose, contemporary 9-12Cr martensitic steels, designated P91, P92, E911, VM12-SHC, and SHM12, were developed . The paper analyzes experimental research on the behavior of commercial ferritic steel P91 samples. Behavior data on the heat affect zone (HAZ) of welded joints were obtained by testing smooth specimens produced by simulation of the new material welding. A survey of testing hardness and tensile properties is given and images of the fracture surfaces of the samples were processed. Image analysis with ImagePro Plus confirmed experimental testing results for the grain size measured by the circle method and a comparative relationship between the percentage of carbides, precipitates, and lamellar diameters with respect to temperature is provided. The material that was PWHT (post-weld heat treatment) has the highest mean value of lamellar diameter, which indicates increased toughness and decreased hardness compared to BM and material without PWHT i.e (The higher lamellar diameter, the smaller material hardness).


Introduction
Thermodynamic efficiency of steam turbine in electricity production in fossil fuel power plants can be increased if temperature and steam pressure are increased at turbine inlet.That is why steels operating under harder conditions are developed to meet the requirements of efficient electricity production.
The material used in this work is commercial ferritic steel P91 that is used for manufacturing the process equipment components in fossil fuel power plants operating under supercritical steam conditions (steam temperature 600 ºC and steam pressure up to 290 bar).Testing involved the behavior of welded joints of the steam line components manufactured of the material P91.Behavior data on HAZ of welded joints were obtained by testing smooth specimens produced by simulation of the new material welding (Pandey et al., 2018;Burzić et al., 2008;Burzić & Adamović, 2008;and Shibli & Hamata, 2001).The paper presents the results of testing hardness and tensile properties, carried out in previous research, as well as the processing of images of the samples' fracture surfaces.Measured lamellar diameters, percentage of carbides and precipitates were compared depending on whether or not additional heat treatment has been done, and conclusion was drawn on material hardness.Also, comparison was made between the grain size measured by the circle method and the grain size measured using ImagePro Plus program.

Experimental
Using the Gleeble system to simulate of the heat-affected zone, thermal fatigue, many other metallurgical studies and the test hot ductility properties.Samples are placed between two sets of jaws that are part of a high current circuit.As current passes through the sample, it is resistively heated.A thermocouple welded to the center of the sample is used in a feedback loop to monitor temperature and control current (Nippes and Savage, 1949).A sketch of the jaws, sample, and thermocouple wires is shown in Figure (1).The data on behaviour of the heat-affected zone (HAZ) of the welded joints were obtained by testing of smooth specimens made by simulation of welding of new material.The aim of performed experiment was to determine hardness values and tensile properties of the sample of base metal, designated BM in this work, and two samples simulated at maximum temperature of 925 ºC, so as to obtain the microstructure of characteristic HAZ.After simulation of the welding process, one sample was subjected to additional heat treatment and
Testing was conducted on a pipe Ø320 mm, wall thickness 14 mm, which ensures conditions of plane stress state during testing, length 140 mm, manufactured of steel whose chemical composition, Table (1), was determined at the Laboratory for quality assurance, US Steel Serbia.

Mechanical properties
The tensile test method provided information on mechanical properties of the tested metal.Material hardness and tensile properties were tested according to standard code.

Tensile properties
Table (3) shows tensile properties of all three samples at operating temperature (ISO E., 2012).Figure ( 2) shows the obtained corresponding diagrams in the stress-deformation system.We note from the stress and elongation curve between the base metal and the tested samples.The highest value of the tensile stress and yield point at without PWHT sample while it is lower at the PWHT.2), we are observing an increase in elongation with stress constant, i.e. the material is not subject to Hook's law, F(x)=K(x) the material cannot recover its original shape after force is removed from it.

Image Analysis
The samples of characteristic states 925 ºC with PWHT, 925 ºC without PWHT and BM were prepared for transmission microscopy, then imaged and analyzed at the Laboratory for transmission electron microscopy, Belgrade University, Faculty of Technology and Metallurgy.

E-92
ISSN (Print): 2413-5267 ISSN (Online): 2706-9966 The obtained images were subjected to analysis in ImagePro Plus program (Vuksanović et al., 2018;Sun & Liao, 2016;and Liang et al., 2015).The microstructure of base material is shown in Figure (3).ImagePro Plus software was also used to measure the grain size and it was compared with the manual grain size measuring method, based on the circle method "Metallography", Schumann.Table (5) shows mean grain size after ASTM E112 "Standard Test Methods for Determining Average Grain Size" (ASTM E112), mean grain size measured by the manual circle method and mean grain size measured using ImagePro Plus program for digital analysis.Comparative relationship between these two methods is also presented in Figure ( 6).

Discussion
When testing to get mechanical properties, a reduction in the hardness of the material post heat treated is noticeable.It is evident from Figure (2) that additional heat treatment has increased the capacity of plastic deformation prior to fracture.Also, by additional heat treatment for HAZ, the tensile properties approached tensile properties of the base material.
It can be seen in Table (4) and Figure ( 5) that mean value of lamellar diameter is the highest for the case of the sample 925 ºC with PWHT, then successively decreases for BM and 925ºC without PWHT.Also, mean value of carbides and precipitates percentage, Figure (4) is lower for the sample without PWHT than for the sample with PWHT.It can be also noted that hardness value is the lowest for the case of the highest lamellar width.
In grain size measurements the deviations are very small, Figure ( 6) therefore it can be concluded that ImagePro Plus method applied for tested samples confirms the results obtained by standard laboratory method, which evidences that such method can be used for the analysis of steel microstructure characteristics.

Conclusion
The presented experimental results indicate the necessity for application of post weld heat treatment (PWHT) on each welded joint of highly loaded part of the steam line that is installed in the components in fossil fuel power plants, because:  In samples of the same material post heat treated (after welding), hardness is smaller compared to the sample without additional heat treatment. Tensile properties indicate that the material post heat treated has higher toughness compared to the material without post heat treatment.Image analysis leads to the following conclusions:  The material that was post heat treated has the highest mean value of lamellar diameter, which indicates increased toughness and decreased hardness compared to BM and material without PWHT.(The higher lamellar diameter, the smaller material hardness). Also, the percentage of carbides and precipitates is higher in the sample with PWHT than in the sample without PWHT, which leads to the conclusion that carbides have positive effects on material toughness. By increasing the percentage of carbides and precipitates, the hardness decreases but toughness increases. ImagePro Plus method can be applied for the analysis of steel microstructure characteristics.

Figure 1 .
Figure 1.Specimen thermocouples welded at center after thermal simulation

Figure 2 .
Figure 2. Comparison of BM tensile properties and tested samples at operating temperture

Table 1 .
Chemical composition of tested material (weight %) Hardness value measured at 600 o C on the BM sample, 925 o C without PWHT and 925 o C with PWHT are presented in Table (2).

Table 3 .
Tensile properties of tested material

Table 4 .
Mean values of the lamellar diameters, percentage of carbides and precipitates

Table 5 .
Grain size measurements in ImagePro Plus program