Understanding and Measuring Decarburization Understanding the forces behind decarburization is the first step toward minimizing its detrimental effects. Decarburization is detrimental to the wear life and fatigue life of steel heat-treated components. This article explores some factors that cause decarburization while concentrating on its measurement. In most production tests, light microscopes are used to scan … Read more
Metallographic Examination of Welds
Why Do We Need Standards?
- Standards are prepared by experts on the subject and they represent a summary of the current state of knowledge on the best practices to use.
- If everyone uses the same, best method then results between laboratories will have better reproducibility (i.e., agreement between laboratories)
- National and international standards are continuously revised and kept up-to-date with changes in technology.
Types of Standards
- Company standards relate best to the local needs, but the methods may be poorly developed and defined
- National standards developed by industry/academic experts in the particular field but the practices and quality of the standards vary from country to country
- International standards –these are written, and balloted in a number of countries with leading technology and should represent the best ideas on a given test method (ISO)
- ISO standards tend to be short in length and contain only the basic information required
- ASTM test method standards give more background details and have precision and bias data based on interlaboratory test programs (“round robins”)
During the past ten years there has been an exponential growth in the power of personal computers and work station computers as well as vast improvements in high resolution cameras and monitors. Image amalyzers have become much more powerful, much faster, fully automated and more user friendly with software customized for many applications.
Following a historical review of methods that have been employed for measuring the interlamellar spacing of pearlite, these methods are compared with reference to a continuously cooled AISI 1040 carbon steel specimen. Extensiveana analysis using TEM thin foils and stereological analysis of TEM replicas, were conducted to determine the mean true spacing. Pearsall’s partial resolution method was rigorously controlled. The TEM method of searching for the finest pearlite colony as an estimate of the true spacing produces variable results with no fixed relationship to the mean true spacing. The analytical method of Roosz et al. for developing the distribution of true spacings produced reasonable agreement with the stereological estimate of the mean true spacing.
The measurement of grain size in annealed face-centered cubic (fcc) metals that exhibit annealing twins is complicated by etching problems and by the presence of the twin boundaries. In a few cases, one can successfully etch certain fcc metals developing only the grain boundaries [1,2]. Such techniques are very useful but have not been developed for most fcc metals and alloys.
Grain Size Measurement
Types of Grain Sizes
- Non-twinned(ferrite, BCC metals, Al)
- Twinned FCC Metals (austenite)
- Prior-Austenite (Parent Phase in Q&T Steels)
Intercept Grain Size Example: Single Phase Twinned Grain Structure
The 100X micrograph is that of a twinned FCC Ni-base superalloy, X-750, in the solution annealed and aged condition after etching with Beraha’s reagent which colored the grains. This is a much more difficult microstructure for intercept counting. The three circles measure 500 mm and P is 63 (intersections with twin boundaries are ignored).
Measurement of Grain Size
Image analysis measurements using a stereological approach have been performed on the inclusions in VIM/EBR, VIM/VAR, and VIM/ESR material from the same parent VIM heat of M-50 bearing steel. Tests made on billet samples produced excessively high, inaccurate results due to very fine microshrinkage cavities associated with the carbide due to insufficient hot reduction. Subsequent tests on bar specimens revealed a closer correlation with the calculated volume fractions based on sulfur and oxygen chemical analysis. However, the measured area fractions were consistently lower than the calculated values. Details of the metallographic preparation problems are provided.
Introduction to Stereology and Quantitative Metallography – Volume Fractions and Spacings