High chrome white iron, or ASTM A532, can offer superior abrasion resistance in applications requiring sliding or gouging. To maximize a high chrome white iron casting’s effectiveness, however, it is critical to design with consistent wall thickness.
Why Wall Thickness is Important
Reason 1: Solidification and Cooling
Varying wall thicknesses can lead to solidification and cooling rate challenges in the casting:
- The thicker sections will need to be supplied feed metal during solidification
- The junctions between thick and thin sections will be subject to porosity and higher stress levels
- A thin section can act as a “cooling fin,” and draw heat away from a thicker section in a manner that is difficult to control
- The casting might come out of the sand after cooling to room temperature, and have detrimental stresses built up in it
- Different section thicknesses in a chrome white iron casting can also have different microstructures, (pearlite, martensite, austenite), which adds to the internal stresses
Reason 2: Processing
Subsequent processing, from shotblast to grinding to heat treatment, can exacerbate these stresses and cause cracking, as can aggressive handling.
Processing these castings carefully becomes an important task for the foundry.
Reason 3: Heat Treatment
Heat treatment also becomes a challenge for parts with drastically different section thicknesses. This is because allowing one section of the casting to cool faster than another can cause extreme build-up of stresses due to microstructural changes. Expertise is needed to ensure that the parts heat and cool in a uniform manner.
Changing an Existing Part to Chrome White Iron
Older parts are occasionally changed from gray or ductile iron to high chrome white iron in order to extend the life of the part. Keep in mind that it may be difficult to change the existing pattern to get the wall thicknesses consistent. This makes design assistance particularly important. A part with different sized wall thickness sections will still be possible to make, it will just take more time and care throughout the process and, as a result, be more expensive.
Read: To learn more about designing with chrome white iron read “Top 5 Design Mistakes in High Chrome White Iron Castings”