Estimation of Ductile-Brittle Transition Temperature Using Small Punch Test in Aged Gas Pipes

Temperature plays an undisputable role in material failure, particularly in fracture mode. From the Second World War and lots of failures in cargo ships due to cold environments, it was figured out that the fracture energy in materials drops by decrease in temperature. This drop occurs at a certain temperature called Ductile to Brittle Transition Temperature (DBTT). This phenomenon is very important in ferritic steels [1], and finding the DBTT is considerably important to designing products and predicting future failures [2,3].

DBTT is important in all industrial structures to estimate the fracture properties and failure. This topic has attracted engineers to predict and postpone pipe failures, particularly gas pipes that experience low temperatures and are prone to catastrophic bursts. Hence, various efforts were made to determine the DBTT in pipes. With this regard, for example, Samal et al. [4] found the master curve in ferritic grade low-alloy steels used in pressure vessels. The steel was highly degraded due to irradiation embrittlement and material aging. The DBTT was also focused on pipeline weld areas [5]. In this study, the Charpy method estimated the DBTT in the affected zone area in the girth welded joints. In another study, Shang et al. [6] proposed a quantitative expression for DBTT in pipeline systems. They collected data and employed machine learning methods to derive the relation for the DBTT. The SPT is test method that was introduced in the 1980s [7] is popular due to the small size of the specimen. This test is employed for material characterization from various points of view, namely determination of mechanical properties such as yield stress [8-10], ultimate stress [11-13], fracture properties [14,15], fatigue [16,17] and creep [18-20].

The SPT is also used for the evaluation of DBTT which is an essential parameter in the field of fracture mechanics. DBTT measurement is conventionally done by Charpy V-Notch impact test (CVN), while it needs a considerable amount of material and a considerable effort for specimen preparation and testing. Hence, the SPT is a favourable alternative for the CVN. SPT is able to determine the DBTT [21], but the DBTT (T_sp) obtained from SPT is considerably lower than obtained from the CVN (T_CVN) [22,23]. However, there have been numerous successful studies to draw a relationship between T_sp and T_CVN, leading to a linear relation between these two temperatures [24-28]. In this linear relation, the T_sp is simply determined by multiplying a factor in T_CVN. This factor was reported around 0.4 in the literature [29,25,30]. Additionally, other quadratic nonlinear relationships have also been reported [31,32]. The current study aims to determine the DBTT in the stainless-steel gas pipe system. The pipe experienced degradation in mechanical and decrease of fracture properties due to damages caused by erosion, hydrogen affect, and other factors during the long service time. To this end, first, a few SPT specimens were removed from the inner surface of the gas pipe and were then prepared and tested at various temperatures from -196 to +50 degrees. These temperatures are achieved by adding liquid nitrogen to the SPT setup located in a chamber. T_spwas determined where the fracture energy is in the middle of the upper and the lower shelf.

Gas pipes are prone to material degradation due to contact with gas, environmental effects, mechanical and temperature fatigue effects, aging problems and others. With this regard, hydrogeninduced degradation is of the essence in all pipeline system components due to the contact of material with natural gas [33]. Hydrogen may be taken up by carbon steels, leading to various degradations, and material embrittlement is their most important mode. Hydrogen causes considerable loss in steel ductility, particularly in the presence of stresses [34]. Moreover, hydrogen can decrease fracture toughness and speed up crack growth [35]. Hence, studying material fracture toughness after degradation is significant in gas pipe systems. This study aims to determine the DBTT of the pipe material, which is stainless steel, used for natural gas pipeline systems. The pipe has been in service for a long time, around 20 years, and has been under hydrogen and loading effects during this time. Thus, the material properties have been changed due to degradation. Thus, the material DBTT should be studied because pipes experience various temperatures due to changes in natural gas pressure. To this end, SPT specimens are extracted from the inner surface of a gas pipe (Figure 1). By the SPT method, the pipe does not experience severe destruction and the area that the specimen was extracted from can be repaired with a patch. The chemical composition of materials is shown in Table 1. XRD is also used to determine material phase fractions (Figure 2). According to this result, around 45 percent of the material is a martensitic phase. The SPT specimen is a disc with 10mm or 8mm diameter and 0.5mm thickness. Figure 3 illustrates an SPT setup in which the specimen is clamped by two dies. The force is applied by a puncher to a 2.5mm ball that is penetrated to the specimen. The result is a force-displacement curve whereby mechanical properties are determined through correlation with standard tests. Figure 4 shows the setup with a chamber and temperature controller to hold the temperature steady. There are two steps for T_sp determination. The first is to calculate the fracture energy from force-displacement curves at different temperatures.

Figure 1:The inner side of the gas pipe.

Figure 2:XRD result for phase fraction determination.

Figure 3:SPT setup and specimen.

Figure 4:SPT setup with cooling chamber.

Table 1:Chemical composition of material.

This energy is calculated by the following equation:

where the F(u) is the force applied to the specimen at displacement represented by ‘u’. uf is the displacement where the force reaches to the maximum force [29,36], or passes it and reaches 80% of the maximum force [37]. E_sp drops at DBTT and two shelves of energy are seen. The upper shelf before DBTT and the lower after that. The DBTT is the average of temperatures related to these two shelves. To reach a better comparison between the two shelves the fracture energy is normalized by the following equation:

Where T is the test temperature and En is normalized energy. This equation divides the Fracture energy to the temperature. This process can help the scientist to achieve a better contrast between the upper shelf and the lower one, simplifying the estimation of the DBTT [38].

SPTs were performed at various temperatures from -196 to 50°C by a setup working based on liquid nitrogen. To achieve more accurate results, three specimens were tested for each temperature. Force-displacement curves are shown in Figure 5. Obviously, the maximum force, failure displacement and fracture energy decrease with temperature reduction. Figure 6 shows the failure displacements at each temperature. Clearly, at 25 °C temperature and 50 °C the fracture displacements are above 2mm. In -10 °C the number comes under 2mm, followed by the second reduction to 1.5mm for -40 °C. The reduction gradually continues to more than 1mm at -130 °C, -160 °C and -196 °C. Fracture energy (E_sp)and normalized fracture energy (E_n) at various temperatures are shown in Figure 7 & 8. Taking the average of upper and lower shelves as the transition point, which is illustrated by two horizontal lines, it occurs at a temperature near -40 °C. Using this temperature and what is reported in the literature the T_CVN can be estimated. The fracture energy is approximately 0.8mJ/N. Figure 9 shows the fracture area in SPT specimens with colours representing the temperatures. Obviously, in 25 °C and 50 °C there is a 2mm height hill and fracture occurs at the hillside [39]. The fracture surface at 50 °C is shown in Figure 10. Spalling is clear at the fracture surface, meaning that the failure is ductile. Additionally, Figure 11 shows the fracture surface at -196 °C. In this case, the fracture is obviously brittle. It is also obvious from Figure 8 that the fracture occurs at small deflection due to the brittleness.

Figure 5:SPT force-displacement curves in various temperatures.

Figure 6:Failure displacements at various temperatures.

Figure 7:Fracture energy measured at various temperatures from SPT curves.

Figure 8:Normalized fracture energy.

Figure 9:SEM images from fracture areas in various temperatures.

Figure 10:Fracture surface at 50 °C.

Figure 11:Fracture surface at -196 °C.

The study focused on a DBTT measurement in stainless steel from a damaged gas pipe. To this end:
a. SPT Specimens were removed from the pipe inner surface.
b. Material composition was first determined by material chemical analysis.
c. The austenitic/martensitic phase fraction was determined through the XRD technique
d. After specimen preparation, the SPT tests were performed at various points from -196 °C to 50 °C, and the forcedisplacement curves were plotted.
e. Fracture displacements and fracture energies at various temperatures were calculated by the area under forcedisplacement curves.
f. Using normalized fracture energies, the DBTT temperature was calculated.
g. Finally, the fracture surface was investigated using SEM images, resulting in a clear transition from ductile to brittle.

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