Determining the Amount of Oxygen, Nitrogen, and Hydrogen in Iron, Steel, Nickel-Base, and Cobalt-Base Alloys

Introduction

In materials such as steel, iron, cobalt- and nickel-based alloys, determining the amount of hydrogen, nitrogen, and oxygen is one of the most important quality metrics. As a general rule, oxygen is utilized to produce steel from pig iron and this is done by getting rid of excess amount of carbon. In addition, oxygen content has to be maintained so as to limit the amount of carbon monoxide that can occur during the solidification process and which in turn can promote extreme porosity.

Nitrogen is a useful alloying agent, but is considered to be impure. Increased amount of nitrogen tends to boost tensile strength and yield, and therefore reduces both formability and ductility. High nitrogen content during solidification may again cause porosity. Increased amount of hydrogen is the major cause of embrittlement, flaking and blistering because of high mobility of hydrogen via the lattice and does not offer much alloying benefits.

ONH836 Instrument

A high-power electrode furnace is used by the ONH836 instrument to discharge the target gases easily and quickly from the sample. The sample enables a quick and concurrent determination of hydrogen, nitrogen and oxygen simultaneously.

The ONH836 instrument by Leco Corporation.

Sampling and Sample Preparation

Both sampling and sample preparation is a vital factor since conventional techniques employed to acquire samples for nitrogen and oxygen determination vary from those suggested for hydrogen, particularly when sampling molten metal. The primary difference in steel sampling processes for hydrogen, oxygen and nitrogen is because of hydrogen¡¯s mobility. Hence, extreme precautions have to be adopted when hydrogen is being sampled.

Whether it is molten iron or steel, a sample should be instantly immersed in cold water and then cooled in a refrigerant like liquefied nitrogen or a combination of solid carbon dioxide and acetone to reduce the rate of hydrogen loss during diffusion.

To determine the concentration of nitrogen and oxygen, a sample that is taken for hydrogen and cooled in a refrigerant can also be utilized. However, a sample taken to determine the amount of nitrogen or oxygen is not apt for determining hydrogen and this is because of hydrogen loss during diffusion. Impurities on the surface have to be removed by either filing or light grinding, and care must be taken to ensure that the sample is not overheated. Consequently, the prepared sample is cleaned in acetone and then dried with warm air. However, the prepared sample should be examined instantly. ASTM E 1806 and ISO 14284 are sampling and sample preparation documents that are specifically meant for iron and steel and provide a good source of information.

Accessories

776-247 Graphite Crucibles, 611-351-181 Lower Electrode Tip for 776-247 Crucibles with automation, and 611-351-182 Lower Electrode Tip for 776-247 Crucibles without automation.

Calibration

501-646, 502-416 steel pin samples; NIST or other appropriate reference materials

Method Selection

In this application note, two techniques are discussed in detail. Both techniques can be utilized to assess steel, iron, cobalt- and nickel-based alloys. For general use, the Precision Method can be employed which will offer optimum accuracy right through the oxygen, nitrogen, hydrogen concentrations seen in this group of metals. The estimated cycle time is 3.5 minutes.

The Fast Track method can prove ideal when quick analysis is required. For instance, when sampling molten metal, results are needed instantly. This technique will create appropriate results for most kind of samples. The estimated cycle time is 2.25 minutes.

Conclusion

As discussed above, both sampling and sample preparation are important for precise determination of the oxygen, nitrogen, hydrogen. Users can determine which technique is suitable to meet their requirements.