Imagine machining the perfect part for a critical application — only to discover the source material wasn’t adequate. The secret to avoiding such a pitfall may lie in understanding steel cleanliness. But what does that truly mean?
What is Steel Cleanliness?
Steel cleanliness refers to a steel’s purity, which is characterized by inclusions, like non-metallic particles, that occur during the steelmaking process. Minimizing inclusions is key to achieving clean, high-quality steel.
High cleanliness in steel translates to higher confidence in its performance, affecting factors such as fatigue strength, corrosion resistance, and overall material integrity. As Todd Chavanne, Business Development at Sullivan Steel, says, “Almost every property is improved by producing a cleaner steel.”
Cleanliness standards vary depending on industry requirements. For highly demanding applications, cleanliness may be defined in an industry specification, like an AMS standard. In the high-performance motorsports sector, steel grades that need to conform to torque sensor standards must meet specific cleanliness requirements.
Steel Cleanliness Standards: ASTM
Nearly all the steel grades Sullivan sells are tested by the mill for cleanliness, typically according to ASTM E45, Standard Test Methods for Determining the Inclusion Content of Steel.
The ASTM E45 steel cleanliness standard provides guidelines for cleanliness testing. It classifies inclusions into four types based on chemical composition: sulfide, alumina, silicate, or globular oxide. “Each different inclusion and size of inclusion has a particular impact on the performance of the steel,” says Todd.
ASTM E45 describes macroscopic and microscopic testing methods to measure the size, shape, and distribution of inclusions. Results from non-destructive testing such as magnetic particle inspection, for example, combined with microscopic testing to classify inclusions, leads to the creation of a JK chart.
JK chart for XD16NW, from Aubert & Duval XD16N brochure
Methods to Improve Cleanliness
Enhancing steel cleanliness is a continual pursuit. Vacuum Arc Remelting (VAR) is one approach to improving cleanliness to achieve performance characteristics. Another is through Electroslag Remelting (ESR). Both offer a potential solution for customers who prefer not to alter their current steel grade but require higher cleanliness.
Specialty grades with high inherent cleanliness are also worth considering. For example, air-melted XD16N has a cleanliness significantly better than remelted 440C.
Image via Aubert & Duval XD16N brochure
As Todd Chavanne knows, “When a grade is brand new to somebody, that's one of the things they're going to check every time — what's the cleanliness? And how are you achieving that cleanliness?”
Steel Cleanliness Misconceptions
There are two notable misconceptions surrounding cleanliness. First, there’s the belief that the cleanliness improvements you get from remelting are identical for all grades, but this is incorrect. The effects of remelting on cleanliness vary from grade to grade. The amount of improvement in steel cleanliness from remelting depends on the specific grade, as some grades exhibit significant improvement while others show less dramatic results.
Another common belief is that remelting a second time will be just as effective as the first time. However, additional remelts yield diminishing returns. “Every time you remelt steel, it is going to get cleaner. But if you remelt a second or even a third time, you're not going to get the same bump in cleanliness as you did the first,” notes Todd. If a remelted grade, particularly one that is VIM/VAR is not clean enough, it’s time to start considering another, better grade entirely (as exemplified by switching from 440C to XD16N).
Finding Steel to Meet Your Cleanliness Needs
At Sullivan, we recognize that cleanliness is just one factor in selecting steel, which is why our experts are here to help. Whether it's deciding on a remelt or exploring a different grade altogether, we provide specialty steel grades and the technical guidance you need to achieve your performance objectives.