Q How can we reduce the variability in results from Brinell hardness testing on quench and tempered steel castings?

The Brinell method of checking hardness is still the preferred method for steel castings and can be very accurate if performed consistently and with proper precautions. Because there are potential metallurgical and processing issues that have an effect on Brinell hardness values, metalcasting facilities must establish their Brinell process capabilities to determine if customer hardness specification ranges can be met.

For example, the range specified by the customer may be too narrow to be consistently achieved. If mechanical property requirements, such as impact properties, are part of the specification, the facility should aim to the lower end of the hardness range. If tensile tests are specified, the casting hardness range may need to be narrower to achieve the strength and ductility requirements.

Brinell testing is a manual process, and the results may vary among different operators and equipment. Equipment should be calibrated every three months, ensuring that the scope is reading accurately, the load being applied is consistent and the steel ball has not been worn or deformed.

Get Consistent

One of the most common causes of test variation results from the Brinell Hardness Test technique employed by the operator. The following points may help improve consistency:

Surface Preparation--How the cast surface is prepared is critical to consistency. The area for the hardness test has several criteria that must be met for an accurate test. It requires removal of the decarburization surface layer, as well as the maintenance of a flat and smooth area for the hardness impression. The area needs to be sufficiently away from the edge of the casting and also thick enough to support the load applied during the test without deforming the casting or cracking the side wall or backside of the casting. Finally, the backside of the area to be tested must be parallel to the testing surface.

The addition of a small circular pad 0.5 in. (12 mm) in diameter and approximately 0.100-0.125 in. (2.5-3.2 mm) high can be effective in reducing variability resulting from testing technique. Grinding the pad flush with the surrounding cast surface removes the decarburization layer, and the resulting flat area eliminates the distortion of the impression caused by excessively concave surfaces when viewed through the Brinell scope. Another benefit is the hardness is evaluated in the same location, eliminating another potential source of variation, particularly with larger castings. The value of this last benefit cannot be overstated; all castings should have a consistent location for hardness testing. Even a uniformly shaped casting can have a consistent location in relation to the in-gate and risers.

Equipment--There are many types of scopes available for viewing the Brinell impression, but it is recommended that you use one that has sufficient magnification to clearly show the graticules markings. The scope also should have a base large enough to ensure stability and a light source.

Guage R&R--Probably the biggest variable in Brinell hardness testing is the human element. There may be tremendous variation in the Brinell measurements from one operator to another. It is recommended that gauge repeatability and reproducibility experiments be conducted to ascertain the measurement variation at metalcasting facilities. As an initial step, a simple program can be devised where all the Brinell operators read an impression on several different castings. The results are recorded and the test repeated on a different day. The next step may be to determine the variability in the process of grinding and taking an impression. Such a test could be conducted on test blocks of various hardness ranges. Operators would grind adjacent sections of multiple test blocks and take Brinell impressions.

Process Variables--Another source of variation is a result of process differences during heat treatment and chemistry variations. Chemistry variations in the casting can result from chemical segregation as the liquid metal solidifies. This normally can be controlled by adequate riser heights. However, it is not unusual that the riser contact areas of steel castings exhibit slightly higher hardness than other parts of the castings due to alloy enrichment at these locations. Inconsistencies in the heating and cooling of the castings may result in inconsistent hardness readings. Ensure that the castings are heated in certified furnaces with good temperature uniformity. The quench tank should have good agitation and sufficient makeup water to reduce the temperature rise in the tank. Ensure that castings being quenched are positioned in the tank in such a way that there are no air pockets (which could result in slower cooling sections and lower hardness values). The steel alloy composition should be designed such that the required hardness is met at the machined surfaces of the castings (rather than only on the as-cast surfaces). In general, this means richer alloy content, especially with elements such as manganese, chromium, molybdenum and possibly nickel.