Why does the hardness of H13 material fluctuate after tempering?

Inaccurate temperature control during the tempering process, uneven cooling resulting from variations in workpiece dimensions, and variations in raw material composition are the primary causes of the hardness fluctuation of H13 material following tempering. Together, these elements have an impact on the martensitic transformation's stability and homogeneity.

 

1. Examination of the Principal Influential Elements

A factor

An explanation

Temperature Deviation Tempering

Stress release is insufficient below 550°C, leading to increased hardness; above 600°C, the hardness diminishes too quickly. A change in hardness of 1~2 HRC can result from a temperature variation of ±10°C.

Variations in cooling rates

The workpiece's thick and thin sections cool at different rates; the thinner areas cool more quickly and have higher hardness, while the thicker sections cool more slowly and are more likely to have soft spots.

Variations in the composition of raw materials

The phase transformation temperature will change and tempering stability will be impacted by variations in the content of alloying elements like Cr, Mo, and V that are more than 0.1%.

Inadequate cycles of tempering

When residual stress is only partially released after one tempering cycle, the microstructure becomes unstable and the distribution of hardness becomes uneven.

Real-world example: A batch of H13 nozzles encountered localized temperature decreases as a result of a tempering furnace temperature control system breakdown; testing revealed hardness variations ranging from 46 to 53 HRC, exceeding the permitted limit by 22%.

 

2. How might hardness variations be minimized? Strict Temperature Control: To guaranty a furnace temperature differential of ≤±5°C, use a tempering furnace equipped with PID temperature control and multi-point temperature measurement;

Standardized Loading: Prevent stacking, maintain consistent workpiece spacing, and guaranty efficient hot air circulation;

Double Tempering: To avoid stress rebound, cool to ambient temperature following the initial tempering;

Raw Material Pre-inspection: Use XRF to verify that the composition complies with GB/T 1299-2018 standard before to heat treatment, paying special attention to Cr (5.0%~5.5%), Mo (1.1%~1.75%), and V (0.8%~1.2%).

Advice: To track the real temperature variations in each furnace, select a service provider that offers furnace temperature profile recording.

 

3. Suggestions for On-Site Reaction

Hardness testing and sampling for every batch: Perform multi-point testing on workpieces at several points using a portable hardness tester, then calculate the average value;

Creating process records: To make problem tracing easier, note each batch's tempering temperature, time, cooling technique, and measured hardness;

CQI-9 certified service providers are given priority because of their stricter process control and greater assurance of hardness uniformity.