Why is there a difference in hardness after tempering in H13 material?

Uneven tempering temperature, irregular workpiece cooling, variations in raw material composition, and insufficient tempering cycles are the primary causes of the variation in hardness following tempering in H13 material. Together, these elements have an impact on martensitic tempering's consistency and stability.

1. Examination of Key Influencing Elements

Factors and How They Affect Hardness

Inaccurate Tempering Temperature Control: The matrix softens and the hardness drops (<48 HRC) at 600°C; tempering is insufficient below 550°C, leading to a higher hardness (>52 HRC). Temperature variations in the furnace surpassing ±5°C can result in significant variations in hardness.

Inconsistent cooling rates: While thicker or stacked parts cool more slowly and readily develop soft spots, thin-walled areas cool fast and have high hardness. This is particularly evident in complicated mold constructions.

Deviations in raw material composition: Workpieces from the same furnace will exhibit inconsistent tempering responses if the phase transformation temperature is altered by variations in the content of alloying elements like Cr, Mo, and V exceeding 0.1%.

One tempering cycle is insufficient to completely release residual stress, which leads to an unstable microstructure and a propensity for stress rebound during further processing or service, resulting in an uneven distribution of hardness.

Real-world example: The actual temperature in a tempering furnace was 15°C lower than anticipated because of a sensor fault. Melt leakage happened within three days of production, and a batch of H13 nozzles showed hardness ranging from 46 to 53 HRC, exceeding the requirement by 22%.

2. How can I recognize and confirm? Multi-point hardness testing: A single-piece hardness difference greater than two HRC is deemed abnormal and necessitates process analysis;

Metallographic analysis: Check for network carbides, untempered martensite, or microstructure coarsening;

Verify the furnace temperature profile to see if there are any pauses or localized temperature variations and whether the tempering temperature is steady between 550 and 600°C;

Composition analysis: To make sure each batch of raw materials satisfies the requirements, use XRF to compare the contents of Cr (5.0%~5.5%), Mo (1.1%~1.75%), and V (0.8%~1.2%).

"Data traceability, reliable microstructure, and controllable process" are essential safety principles.