What are the specific reasons for the large fluctuations in hardness of H13 material after tempering?

Inaccurate tempering temperature control, incorrect workpiece loading that results in uneven cooling, variations in raw material composition, and insufficient tempering cycles are the primary causes of the significant variations in hardness of H13 material following tempering. All of these elements have an impact on how uniformly microstructure change occurs.

1. Analysis of the Primary Cause

Causes Particular Effects

Uneven or Large Fluctuations in Tempering Temperature: Localized areas may undergo over-tempering (hardness < 48 HRC) or insufficient tempering (hardness > 52 HRC) when the temperature differential within the furnace is > ±5°C. A hardness shift of 1~2 HRC can occur for every ±10°C.

Inadequate Furnace Loading Method: When workpieces are stacked or spaced less than five centimeters apart, hot air circulation is impeded, which results in uneven cooling in the "shielded area," creating hard or soft spots.

Inadequate Tempering Cycles (Just One): When residual stress is not completely released, the microstructure becomes unstable and there is a chance that stress could rebound during further processing or service, which causes an uneven distribution of hardness.

Variations in Raw Material Composition: The uniformity of tempering response can be impacted by changes in the phase transformation temperature caused by variations in the content of alloying elements like Cr, Mo, and V > 0.1%.

Variations in Thick and Thin Regions' Cooling Rates While bigger portions cool more slowly and are more likely to have soft spots-particularly in intricate mold structures-thin-walled sections cool quickly and have high hardness.

Real-world example: A company's tempering furnace sensor malfunctioned, causing the actual temperature to be 15°C lower than anticipated. As a result, a batch of H13 nozzles exceeded the specification by 22%, with hardness ranging from 46 to 53 HRC.

2. How can I recognize and confirm?

Multi-point hardness testing: An anomalous single-piece hardness difference greater than two HRC necessitates process analysis;

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

Verify the furnace temperature profile to see whether there are any discontinuities or localized temperature variations, and whether the tempering process temperature is constant 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, organization credibility, and process controllability" are essential safety principles.