As was discussed last month, having the right analysis equipment is one thing; being able to use it effectively is quite another. Several case studies will illustrate the use of the various tools available to testing laboratories and universities and how they help identify the root cause of real-world problems. Let’s learn more.

 

Case Study 1: Gear Investigation

Gears of SAE 5115 steel were failing prematurely. The parts had been low-pressure “vacuum” carburized at 960ºC (1760ºF) and high-pressure gas quenched from 860ºC (1580ºF). The quench pressure was not identified. The gears were submitted to the laboratory for evaluation of the case-hardened layer.

The critical question to be addressed was if the cooling rate used was proper. In order to address this, the following tests were selected: a full metallographic analysis of the case and core regions, including determination of grain size and retained austenite levels, hardness (macro and micro) measurements and a stress profile in the hardened layer.

 

Metallographic Examination

A gear-tooth section was cut from the gear, mounted, ground and polished then evaluated on a standard optical microscope. The sample was etched with a 2% Nital solution.

The microstructure of the case layer was tempered martensite with about 20-30% retained austenite (visual). Surface hardness was 61 HRC. The microstructure of the core was a combination of ferrite, pearlite and bainite. Core hardness was 42 HRC. The grain size was measured as ASTM No. 10. Microhardness testing (513 HV1) revealed an effective case depth of 0.85 mm (0.0335 inch), which met specification.

 

Stress Profile in the Hardened Layer

Testing of residual stress present in the case layer (Fig. 1) indicated this area was in tension, which would explain the premature failure being experienced in the field. This suggested (and was confirmed by a subsequent literature study) that the gear was cooled too rapidly from austenitizing temperature. 

 

Case Study 2: Boiler-Plate Screen Investigation

Pipes installed in the combustion chamber of a steam boiler were found to have failed prematurely (Fig. 2). The material in question was an ASTM A204 Grade-A pressure-vessel steel (16Mo3). These pipes operated at a temperature of 360ºC and a pressure of 14.5 MPa (2.1 ksi). The goal of the investigation was to determine why these components failed.

The following tests were conducted: visual examination, stereographic microscopy, chemical composition analysis, hardness measurements (Fig. 3), metallographic examination, endurance tests, hydrogen analysis and a Baumann micrographic examination (a qualitative test that is employed to detect the distribution of sulfur in steel and certain physical irregularities, such as cracks and porosity, by printing on photosensitive paper previously soaked in sulfuric-acid solution).

 

Test Results

The torn pipe edge was observed to be tough and brittle. The wall thickness was measured and found to be 3.0 mm (0.120 inch), which was below the original design thickness of 3.52 mm (0.140 inch). On the damaged section in the furnace chamber, plastic deformation (swelling) of the cross section was observed and measured at approximately 6.0%. Hardness values (Table 1) on the exposed side of the pipe averaged 136 HV10 and averaged 150.5 HV10 on the protected (brickwork) side. Additional testing confirmed the following:

  • The chemical composition was proper for the material in question.
  • The damage section on the combustion chamber side was found to have a degraded microstructure (Figs. 5-8).
    • Due to prolonged high-temperature exposure above 510ºC (950ºF), the primary ferrite/pearlite microstructure transformed to a ferrite/carbide microstructure. This was accompanied by a reduction in hardness (from approximately 148 to 126 HV10) with an accompanying decrease in strength.
  • In the area of the pipe recessed into the brickwork, the material retained its ferrite/pearlite microstructure. As such, tensile strength and elongation values were found compliant with the requirements of the original material specification.

Analysis showed that the tube material did not pick up hydrogen, but on the side in the combustion chamber the corrosion products adhering directly to the metal saw a significant amount of sulfur present (Fig. 4).

 

Conclusions

The pipe was damaged due to overheating. The pipe exposed to the combustion chamber has the characteristic features of damage from prolonged overheating (estimated to be approximately 1,000 working hours), including swelling, brittle (edge) fracture, microstructural breakdown and loss of strength properties (e.g., tensile strength, hardness).

The cause of this type of damage could only be due to excessive heat or limited/inadequate refrigerant flow. The research excluded the possibility of hydrogen-assisted cracking as a potential cause.

 

Summary

These case studies, and many more like them, show the value of using a number of different analysis tools to find the root cause of a problem or failure so that proper corrective action can be implemented. Picking a laboratory with the type of tools needed to do the job is a critical component to understanding if a heat-treatment problem (case study No. 1) or a field operational issue (case study No. 2) is at fault.


References

  1. Professor Leszek Klimek, Institute of Materials Science and Engineering, Łódz´, University of Technology, technical contributions
  2. Professor Emilia Wołowiec-Korecka, Łódz´ University of Technology, Łódz´, Poland, technical contributions and private correspondence
  3. Herring, Daniel H., Atmosphere Heat Treament, Volume II, BNP Media, 2015
  4. “Charpy vs. Izod: An Impact Testing Comparison,” Element (www.element.com)
  5. Herring, Daniel H., A Comprehensive Guide to Heat Treatment, Volumes 1 & 2, BNP Media, 2018 (e-books)