How to test the hardness of H13 material after standard tempering?

After conventional tempering, H13 material must be tested for hardness using a portable hardness tester at several locations on the workpiece to ensure an average value of 48–52 HRC and a single-piece range of ≤2 HRC. This guaranties accurate and trustworthy data, together with surface treatment and instrument calibration.

 

1. Typical On-Site Inspection Process

Procedures

Important Points

Justification

① Preparing the Surface

Remove the oxide and oil coatings from the test points by sanding them until they have a metallic sheen.

To prevent low readings, make sure the probe makes good contact.

② Calibration of Instruments

Prior to testing, calibrate on a standard block and keep the error under ±1 HRC.

Boost repeatability and stop systematic variations.

③ Test Point Configuration

For every piece, choose three to five crucial points: the center, the edge, and the thickness boundary.

To increase representativeness, cover areas that are susceptible to uneven cooling.

④ Several Measurements

Calculate the average value after measuring each point three times. Adjacent indentations should be spaced at least 3 mm apart.

Improve data stability and lower random errors.

⑤ Analysis of Data

Determine the batch standard deviation and the individual piece range (maximum value minus minimum value).

Determine anomalous workpieces and evaluate consistency.

Pass Criteria: Batch average stable at 48–52 HRC, individual piece range ≤2 HRC.

 

2. Precautions for Inspection

Cool to room temperature: To avoid hot softening influencing the measurements, the workpiece temperature must be less than 50°C before testing;

Prevent edge effects: To avoid deformation diffusion interference, the indentation distance from the edge should be at least 3 mm;

Traceable records: To create a closed-loop quality control system, mark each component with its serial number, inspector, time, and furnace number;

Frequent equipment maintenance: To guaranty precise measurement, portable hardness testers should be examined every three months.

For instance, five measuring spots on an H13 manifold plate had hardness values of 50.1, 50.3, 49.8, 50.0, and 50.2 HRC, with a range of only 0.5 HRC, which is regarded as extremely consistent.

 

3. Additional Verification Techniques

Metallographic Sampling Analysis: To determine whether the microstructure is uniform tempered martensite + scattered carbides, samples of parts with unusual hardness are prepared for observation;

Verify the furnace temperature curve to see whether there are any disruptions or localized temperature variations, and whether the tempering temperature is steady between 550 and 600°C;

Composition Comparison: By removing the impact of material fluctuations, XRF is utilized to verify that the Cr, Mo, and V contents satisfy the requirements.

"Data speaks for itself, multi-dimensional verification" is a safety principle that guaranties the test findings accurately reflect the material state.