Hand Tool Heat-treatment Basic Knowledge

What is the reasonable hardness range?

The hardness value of a material is determined using the Rockwell test (which measures the depth of an indenter under a major load as compared to that made by a minor load) and classified according to various Rockwell scales denoted by the values HRA, HRB, HRC, and so on.

Sometimes customers try to request hand tools of a particular hardness. At times, their requests may be too specific. For example, they might say: Our target is HRC 48; if it’s HRC 47, we consider it to be defective and we will not accept that.

Well, this kind of request would only confuse our engineer.  In mass production, it is technically impossible to meet such stringent specifications. If you’ve ever baked cookies in an oven, you’ll know that no two cookies ever turn out exactly the same. Each portion of cookie dough has a slightly different composition, and the temperature will vary according to where it is positioned within the oven. Similarly, during the heat treatment of hand tools, each item in the furnace will differ minutely from the others in terms of material components, how its temperature is affected by its position, and so on. All of these factors will influence the hardness. 

Furthermore, we have to factor in the tolerance of the HRC inspection machine that performs the Rockwell hardness test. The tolerance of the machine itself may be +/- 1 ~ 2 HRC.

The quote from ASTM E18-15, Standard Test Methods for Rockwell Hardness of Metallic Materials :

3.1.7.2 Rockwell hardness standardizing machine- a Rockwell […] The standardize machine differs from a regular Rockwell harness testing machine by having tighter tolerance on certain parameters.

If someone were to claim they can manufacture tools of a precise hardness, I would be quite curious as to what black box they were using for heat treatment.  When there is a tighter range of tolerance of harness request, the total cost is higher, because it requires not only more precise control on heat treatment, but also the previous machine processes, such as forge, machine, quality material selection, and so on.  Coming back to the original request—I would want to know why the customer needs such precise hardness. Are they building a rocket or an aircraft?

In cases where our customers have a special demand for durability and torque- loading performance, we usually suggest that they consider establishing the reasonable range of hardness by selecting the right material and the right heat-treatment machine, to steadily increase the quality performance in terms of durability and torque loading.

If a part beakes, is it due to low quality heat-treatment?

If a product breaks or malfunctions, it can be tempting to jump to conclusions. Product defects may occur due to poor design, wrong material, insufficient processing or inadequate heat treatment. Certainly, failing to set up the right time or temperature during heat treatment is a possible cause of deficiency.

If defective occured, check individual and then check the production history is how our QC would look. The defective can be infered by inspecting piece to piece among the mass production and checking their production history.  

During the forging process, we apply a process called annealing, which increases ductility and reduces hardness in order to make the material soft enough to forge. However, if the annealing adjustment is incorrectly executed, the material will be excessively carbonized, and the subsequent heat treatment process will cause the material to become excessively brittle and result in poor torsion. In such a case, poorly executed heat treatment is not the culprit.

That's the reason why our QC look closely on material component, forging process and heat-treatment, because these 3 elements are critical to the quality hand tools performance.  If you want to know more about how Tien-I make the quality socket and impact socket for pro brands around the world, please check below link:

"from concept to quality tools"

Tien-i is Taiwan’s leading manufacturer of professional hand tools.  We welcome requests from various field industries and brands who are interested in quality hand tools. If you have any questions, please contact our helpful team of experts.