Thermocouple&RTD Knowledge

Analysis of brittle fracture of platinum and rhodium thermocouple wire in hot blast stove of steel plant

Anhui Tiankang（Group）Shares Co.,Ltd analyzed the platinum-rhodium 10-platinum dual wire of the hot blast furnace in the steel factory with an early breakage by using an electronic probe microanalyzer (EPMA). The results show that the thermocouple protection tube of the hot blast furnace in the steel plant is not tightly sealed, and the ambient atmosphere is in direct contact with the thermocouple couple wire of the hot blast furnace in the steel plant during use. The brittle fracture along the grain is the main reason for the failure of Analysis of brittle fracture of platinum and rhodium thermocouple wire in hot blast stove of steel plant.
Platinum-rhodium 10-platinum thermocouples are the most commonly used thermocouples with high accuracy. In addition to being used as standard internal inserts, they are also used for-temperature standard and second-class standard temperature transfer. They are also commonly used in aviation and aerospace. Temperature measurement. Its long-term use temperature is 1000-1300 ℃, and it can be used at 1600 ℃ in short-term. The use environment is oxidizing or neutral atmosphere, and can also be used under vacuum. A steel factory hot air furnace thermocouple is platinum rhodium 10-platinum (PtRh10-Pt) precious metal thermocouple, the wire diameter is Φ0.5mm, the protective ceramic is alumina, and the protective tube is 3YC52 high temperature alloy tube (the new one can withstand 1300 ℃ Alloy manufacturing). The hot blast stove thermocouple is used in the hot blast stove for steel making. The use temperature is about 1200 ℃. After only about 40 hours of use, it is found that the temperature is abnormal. So the thermocouple was removed and it was broken into several sections after disassembly. Be embrittled. In order to avoid the occurrence of similar accidents, the failure analysis of the thermocouple wire of the hot blast stove was carried out.
1. Test method
After the broken platinum-rhodium 10-platinum pair wire was cleaned with alcohol and ultrasonic wave, the electron shape observation, micro-region qualitative and quantitative analysis of the broken wire and the outer surface of the pair wire were performed with JCXA-733 electron probe microanalyzer Surface and line scan analysis, EPMA use conditions are accelerating voltage 29kV, current 2.0 × 10-8A.
2. Test results of analysis of brittle fracture of platinum-rhodium 10-platinum wire in hot blast stove of steel mill
2.1 Macro analysis
The thermocouple is composed of platinum rhodium 10 and pure platinum wire. The pure platinum couple wire is more damaged than platinum rhodium 10, which has been broken into 7 segments, and platinum rhodium 10 is only broken into 3 segments. Seriously, this is related to the high temperature of the hot end. Observed with the naked eye, the fracture of the pure platinum double wire is more reflective, and the sparkling characteristics can be seen when rotating under strong light. Because the diameter of the even wire is only Φ0.5mm, the pure platinum wire used near the hot end is severely damaged, and then it is placed in a JCXA-733 electronic probe microanalyzer to analyze the surface morphology and fracture.

2.2 Surface analysis of thermocouple wire

$brittle fracture thermocouple$

Surface secondary electron morphology of Pt wire

The typical secondary electron morphology image (Second-ary Electron Image, SEI) of the outer surface of pure platinum dual wire is shown in Figure 1a and Figure 1b. It can be seen from Figures 1a and 1b that although the platinum-rhodium 10-platinum wire does not use grain boundary etching technology to show its grains, its grain boundaries are clearly visible, and the grain boundaries are lower than the matrix and sag. This phenomenon shows that The grain boundary of the silk has been damaged during use.
Qualitative and quantitative analysis of the matrix and grain boundary phases in Fig. 1b, the results are shown in Table 1. It can be seen from Table 1 that the P content of the grain boundary phase has reached as high as 3.51% -5.14%. According to the Pt-P phase diagram, the P content in the Pt20P7 phase formed by P and Pt is 5.26%, so the grain boundary phase may be the Pt20P7 phase.
Figure 1c is an enlarged triangular grain boundary morphology. It can be observed that a new phase may have formed in the grain boundary, and cracks exist, showing brittle characteristics. The surface scan analysis showed that P element segregated at the grain boundary. Figure 1d is the distribution diagram of P element on it. It can be seen that P element has obvious segregation.
Analysis and discussion on brittle fracture of platinum-rhodium-platinum-coupling wire of thermocouple in steel factory hot blast stove
Hot blast furnaces in steel mills are very common equipment used by metallurgical enterprises. They use coal, coal gas or natural gas as fuel. The temperature is usually around 1200 ℃. Platinum and rhodium 10-platinum thermocouples are used to measure the temperature. The furnace is a typical reducing atmosphere , Mainly CO2, CO, O2 and N2.

The platinum-rhodium 10-platinum thermocouple has a brittle fracture failure in a reducing atmosphere, usually for the following reasons: ① Hydrogen atoms can penetrate into the platinum metal at high temperature, causing fracture due to hydrogen embrittlement; ② Carbon can dissolve at high temperature In platinum, after cooling, it precipitates out of platinum in a graphite shape, making platinum brittle and easy to break; ③Platinum and platinum-rhodium alloy come into contact with refractory materials containing SiO2 in a reducing atmosphere at high temperature to produce platinum silicide, causing brittle fracture.

It can be seen from the above test results that the double wire is not caused by the above reasons, but is attacked by P element, and segregation occurs on the grain boundary of the double wire, and the P content of the grain boundary phase has reached 3.51% -5.14 %. According to the Pt-P phase diagram, P and Pt can form a Pt20P7 phase with a P content of 5.26%. Therefore, the P content at the grain boundary of the wire is sufficient to form a Pt20P7 phase. This phase and Pt are easy to form a eutectic, and the eutectic The temperature is only 588 ℃. When using the temperature of 1200 ℃, we can see from the phase diagram of Figure 5 that the metal Pt- coexists with the Pt% of the melt containing P. When the temperature drops, the traces of the melt flow are retained. This can be seen from the figure The grain boundary of 3 shows that the Pt20P7 phase is parallel, and this phase is also relatively brittle, and it is easy to form cracks, as shown in Figure 1c, which eventually causes the even wire to break along the crystal. Therefore, it can be considered that P element is the main cause of broken filaments.