Infrared/thermal conductivity method to detect the content of oxygen and nitrogen in C-103

Niobium is an off-white metal with atomic number 41, belonging to the VB group of the periodic system. Niobium has outstanding advantages such as high melting point, low density, good plastic toughness, good welding performance, and high specific strength. It is a candidate material for new aerospace structural parts used at higher temperatures.

Niobium-hafnium alloy has good high-temperature mechanical properties, processability, and stable chemical properties. It is expected to replace nickel-based and drill-based superalloys for important parts of ultra-high-speed aircraft engines, and has a broad application prospect. C-103 niobium-hafnium alloy is a new type of niobium alloy, containing 89% niobium, 10% hafnium and 1% titanium. The welding performance and formability are better than other niobium alloys, and it can meet the operating temperature of 1480 ℃ rocket engine The need for components is a new generation of alloys for aerospace propulsion systems, which are suitable for the manufacture of liquid rocket engine nozzle extensions, orbital maneuvering engines, and high-plastic attitude control engine radiation cooling thrust chambers.

Oxygen and nitrogen are the main non-metallic impurities in C-103 niobium hafnium alloy, mainly in solid solution or compound state, which easily reduces the impact and fatigue resistance of the metal, and has a greater impact on the performance of C-103 niobium hafnium alloy. Therefore, it is necessary to accurately determine the oxygen and nitrogen content in C-103. At present, the inert gas melting infrared method is generally used for the determination of oxygen in metals, and the inert gas melting thermal conductivity method is generally used for the determination of nitrogen content. This time, an inert gas fusion infrared/thermal conductivity method was used to establish a method for the simultaneous determination of oxygen and nitrogen in hafnium. This method is simple to operate, less sample consumption, fast detection speed, and accurate detection results.

Experimental part 1.1 Apparatus and reagents

Oxygen and nitrogen analyzer; ON736 oxygen and nitrogen analyzer, composed of pulse heating system, infrared/thermal conductivity detection system, analytical balance and computer system. It uses high-purity helium as the carrier gas, and the sample is wrapped in nickel flux. The sampler is dropped into the hot graphite crucible that has been degassed. The oxygen in the sample is reduced to CO by graphite, converted to CO2 by the hot CuO reagent, and then enters the infrared detector along with helium for detection, and compares with the standard substance to obtain the oxygen content; nitrogen is released in the form of N, along with helium The gas enters the thermal conductivity detector and compares it with the standard substance to get the nitrogen content.

Standard material: LECO 501-653 titanium standard material, oxygen content: 0.053% ± 0.003%, nitrogen content 0.008% ± 0.002%. Flux: high purity nickel basket, w(O)≤0.0005%, w(N)≤0.0001%. Nickel basket pickling solution: the ratio is 25 mL concentrated nitric acid + 75 mL glacial acetic acid + 2 mL concentrated hydrochloric acid.

Graphite crucible: spectral pure graphite sleeve crucible, spectral pure graphite standard crucible, spectral pure graphite high temperature crucible.

Carrier gas: helium, with a purity of 99.995%. Acetone: analytically pure.

Sample: Clean surface, uniform C-103 niobium-hafnium alloy wire with a diameter of 3mm.

1.2 Test method

The instrument is turned on and warmed up for 2 hours. After the various parameters are stable, the blank value is measured 3 to 5 times in parallel, and the blank calibration is performed. Then the reference materials of 501-657 and 501-657 were measured in parallel for 3 times, and the instrument was calibrated.

Place the nickel basket containing the C-103 sample in the sample dropper, and drop it into the hot graphite crucible through the sample dropper. The instrument automatically detects and displays the oxygen and nitrogen content.

1.3 Sample processing

The C-103 niobium-hafnium alloy wire sample was processed into suitable specifications, washed with acetone, and dried for later use.

2 Results and discussion

2.1 Treatment of nickel basket flux

C-103 niobium-hafnium alloy has a high melting point, and its molten state has a poor affinity with carbon. In order to ensure the complete release of oxygen and nitrogen in the sample, a flux must be added. The nickel basket is a common flux for detecting oxygen and nitrogen in metal materials. However, uneven and slight oxidation and pollution on the surface will cause the blank value to be unstable, which will have a greater impact on the oxygen and nitrogen content detection of the sample. The nickel basket is pretreated. Place the nickel basket in a beaker containing nickel pickling solution for 60 seconds, take it out and rinse it with a large amount of running water, then wash it with acetone 3 times and then soak it in acetone, and take it out to air dry before use. Use spectroscopic pure graphite sleeve crucible, choose analysis power of 5000 W, and perform blank value detection on untreated and treated nickel baskets. It can be seen that the blank values of oxygen and nitrogen are low and stable after the nickel basket is processed.

2.2 Graphite crucible experiment

Graphite crucible is the reducing agent for oxygen and nitrogen content detection in C-103 niobium-hafnium alloy. Common graphite crucibles for gas analysis include standard crucibles and high-risk crucibles. Using these three types of crucibles, the oxygen and nitrogen in the C-103 niobium hafnium alloy sample were detected. The results can be seen that using these three sets of crucibles, the oxygen content detection value is stable, and the standard crucible and high-temperature crucible are used. The detection value is unstable. The heat preservation effect of the set of crucible is good, the temperature of the inner crucible is stable, and the nitrogen detection result is stable; the standard crucible temperature is not stable enough, the nitrogen release curve is fluctuating, and the detection value is unstable; the high temperature crucible is partially corroded, and the nitrogen detection result is unstable. 2.3 Detection limit and lower limit of quantification

After the nickel basket is processed, it can be obtained from the data that the measurement result of the blank value of oxygen and nitrogen is repeated n (n=7) times. The detection limit of the method is calculated according to the following formula.

MDL = t(n—1.0.99)X S

Where:

MDL: Method detection limit;

n: the number of parallel determinations of the sample;

T: The tail of the degree of freedom is n-1, and the t-distribution when the confidence tail is 99%; S: the standard deviation of n parallel determinations.

After calculation, the detection limit of oxygen is 0.9μg/g, the detection limit of nitrogen content is 0.3μg/g, 4 times the detection limit is taken as the lower limit of quantification, and the lower limit of quantification of oxygen and nitrogen content of C-103 niobium hafnium alloy They are 3.6μg/g and 1.2μg/g, which meet the product requirements.

2.4 Analysis of power and shortest integration time

The temperature of the graphite crucible directly affects the fluidity of the melt and the release of oxygen and nitrogen. The temperature of the crucible depends on the analytical power of the instrument. Therefore, it is very important to select the optimal analytical power. In this experiment, the analysis power of 4000W~5500W was selected to detect the oxygen and nitrogen content of the C-103 niobium hafnium alloy sample. The results show that the analysis power is from 4000W to 5500W, and the oxygen content detection result does not change significantly; the analysis power is from 4000W to 4800W, and the nitrogen content detection result gradually increases; 4800W to 5500W, the nitrogen detection result is stable; that is, the analysis power is 4800W to 5500W Below, the test results of the oxygen and nitrogen content of the C-103 sample are stable. Because high power will increase the energy consumption of the instrument and the loss of components, this experiment chooses to analyze the power as 5000W. The degassing power of 6000W is selected to make the graphite crucible completely degassed. According to the integration chart of ON736 oxygen and nitrogen analyzer, the oxygen signal value returns to the baseline and stabilizes after the integration time of 25s, and the nitrogen signal value returns to the baseline after the integration time of 45s. Therefore, the shortest analysis time selected in this experiment is 55s.

2.5 Sample quality

The different mass ratio of the C-103 sample and the nickel basket flux will affect the fluidity of the melt and the release of oxygen and nitrogen elements, which will have a greater impact on the test results. This experiment selects the sample mass from 0.05g to 0.20g, and detects the oxygen and nitrogen content. The test results can be seen. When the sample mass is greater than 0.15g, due to the melting point and melt flow of the C-103 sample and the nickel basket The situation is different. When the mass of the sample is too large, the contact between the sample and the graphite cyanosis is not sufficient, the release of oxygen and nitrogen is not complete, and the detection value is reduced. In this experiment, the mass of the sample selected is 0.10g.

2.6 Precision experiment

Weigh out 0.100g C-103 sample, the analysis power is 5000W, and the oxygen and nitrogen content are detected 11 times under repeatable conditions. The results obtained show that the RSD of oxygen is 3.36% and the RSD of nitrogen is 6.86%. The precision of this method good.

2.7 Standard addition recovery experiment

Accurately weigh 5 parts each of 0.1000g of the titanium standard material LECO 501-653 and C-103 samples, take one part each of the sample and the standard material in a nickel basket, and then place it in the dropper for the addition and recovery experiment The test results show that the recovery rate of oxygen standard addition is 95.5%~106.0%, and the recovery rate of nitrogen addition standard is 92.5%~107.5%, and the recovery rate is good.

3 Conclusion

The experiment determined the flux treatment method, graphite crucible type, detection limit, lower limit analysis power, and sample quality for detecting oxygen and nitrogen content in C-103 niobium-hafnium alloy samples. Through repeatability and standard addition and recovery experiments, the precision and accuracy of the experimental method were verified. This method can accurately detect the oxygen and nitrogen content in the C-103 niobium-hafnium alloy sample.