How to detect hardness fluctuations in H13 material after tempering?

After tempering, multi-point measurements using a portable hardness tester are the main method used to detect hardness fluctuations in H13 material. Batch data statistical analysis is then used to evaluate the consistency and homogeneity of the hardness distribution. This technique is quick, non-destructive, and appropriate for batch screening on-site.

 

1. Standard Procedure for On-Site Testing

Procedures

Important Points

Justification

① Choose a Test Area

Each workpiece should have three to five testing spots marked on its surface, edges, center, and thickness joints.

covers regions that are vulnerable to irregular cooling.

② Surface Care

Use sandpaper to gently sand the testing spots.

eliminate oil spots or oxide layers to guaranty excellent contact and prevent surface conditions from affecting readings.

③ Measuring Hardness

Use a portable Rockwell (HRC) or Leeb (HL) hardness tester in converted mode. Calculate the average value after measuring each point three times.

enhance the accuracy of the data.

④ Recording and Comparing Data

Note the hardness values at each location, then compute the standard deviation and range (highest value minus minimum value).

ascertain the level of variation.

Acceptance criteria include an average value of 48 to 52 HRC for the entire batch and a hardness fluctuation of ≤ 2 HRC for a single piece within the same batch.

 

2. Risk Level and Fluctuation Assessment

Range of Fluctuation

Judgment Outcome

Suggestion

≤1 HRC

Great, reliable procedure

Direct use is possible.

1-2 HRC

Normal variation, acceptable

Boost the monitoring of processes

2–3 HRC

There are some risks, which require further research.

Metallographic study and sampling to confirm microstructure homogeneity3 HRC

There are unqualified, soft places, or untempered areas.

Rework or discard the whole batch

For instance, when a manufacturer examined a batch of H13 manifolds, it discovered that one part had a local hardness of 46.5 HRC, while the other parts had a range of 3.7 HRC and a local hardness of 50.2 HRC. Uneven cooling was found to be the cause of this soft area issue.

 

3. Additional Verification Techniques

Metallographic Sampling Analysis: To verify the existence of untempered martensite, retained austenite, or microstructure coarsening, samples of workpieces with unusual hardness are prepared for examination.

Verify the furnace temperature curve to see if there are any localized temperature variations or disruptions and if the temperature stays constant between 550 and 600°C during tempering.

Composition Pre-inspection and Comparison: To eliminate the impact of compositional variances, XRF is used to verify the uniformity of Cr, Mo, and V concentrations in the raw materials.

The safety principle "Data speaks for itself, multi-dimensional verification" guaranties that variations in hardness may be tracked and managed.