Precipitation hardening stainless steel (PH stainless steel) is a type of stainless steel whose strength and toughness are improved by precipitating a second-phase strengthening phase through heat treatment. It combines the corrosion resistance of austenitic stainless steel with the high strength of martensitic stainless steel. The core of its heat treatment process is to achieve performance control through two steps: solution treatment and aging treatment (precipitation hardening). The choice of aging temperature directly affects the final microstructure and mechanical properties.

I. Heat Treatment Process Operation Steps for Precipitation Hardening Stainless Steel

The heat treatment of precipitation hardening stainless steel is generally divided into three stages: solution treatment (softening annealing), conditioning treatment (optional), and aging treatment (precipitation hardening). The specific operation is as follows:

1. Solution Treatment

Purpose:
To fully dissolve carbides and intermetallic compounds into the austenitic matrix through high-temperature heating, followed by rapid cooling (quenching) to obtain a uniform supersaturated solid solution, providing conditions for subsequent aging precipitation.

Operating Points:

Temperature Range: Generally 950~1150℃ (slightly varies depending on the steel grade, e.g., 17-4PH is typically 1020~1060℃).

Heating Method:

Use a controlled atmosphere furnace or vacuum furnace to avoid surface oxidation (a protective coating can be applied if necessary).

Holding Time:

Depending on the workpiece thickness, typically 10~60 minutes (longer for thicker sections).

Cooling Method:

Water quenching or oil quenching (rapid cooling is essential to prevent the precipitation of brittle phases).

2. Conditioning Treatment (Optional)

For some steel grades (such as the H1150 series of 17-4PH), an intermediate conditioning treatment is added before aging (e.g., short heating at 800~900℃ followed by air cooling) to optimize the distribution of precipitated phases and improve toughness.

Typical Process:

Heating at 815~870℃ for 10~30 minutes, followed by air cooling or water quenching. 3. Aging/Precipitation Hardening

Purpose:

To slowly precipitate alloying elements (such as Cu, Nb, Mo, etc.) from a supersaturated solid solution by heating at a relatively low temperature, forming fine, dispersed strengthening phases (such as ε-Cu, Ni₃Ti, etc.), significantly improving strength and hardness.

Operating Points:

Temperature Range: 450~650℃ (different aging temperatures are selected depending on the steel grade and performance requirements).

Holding Time:

1~4 hours (depending on workpiece size and temperature, usually 1~2 hours is sufficient).

Cooling Method:

Air cooling (most common), water cooling can be used for special requirements.

Examples of typical aging processes for common steel grades:

17-4PH (0Cr17Ni4Cu4Nb): Standard aging: 480~620℃ (e.g., 480℃×1h air cooling, hardness reaches HRC40~45; 620℃×1h air cooling, hardness approximately HRC30~35).

High toughness option: 550℃ aging, balancing strength and toughness.

15-5PH: Typically aged at 550~600℃.

PH13-8Mo: Aging at 480~560℃, achieving higher strength.

II. Influence of Different Aging Temperatures on Performance

Aging temperature is a key parameter controlling the properties of precipitation-hardening stainless steel, directly affecting the balance of strength, hardness, toughness, and corrosion resistance. The effects are as follows:

1. Low-temperature aging (450~500℃)

Significant strengthening effect: The precipitates (such as ε-Cu) are small and dense, strongly hindering dislocation movement, thus resulting in the highest strength and hardness (e.g., 17-4PH can reach HRC 45~50 after aging at 480℃).

Lower toughness:

Excessively high precipitation density may lead to localized stress concentration, resulting in slightly lower impact toughness. Suitable for high-strength but medium-low toughness applications (such as fasteners and bearings).

2. Medium-temperature aging (500~550℃)

Optimal overall performance:

The precipitate size is moderate, ensuring both high strength (HRC approximately 40~45) and good toughness (high impact energy), making it the preferred choice for most applications (such as aerospace structural components).

Good corrosion resistance:

Compared to low-temperature aging, there is less grain boundary precipitation, resulting in stronger resistance to intergranular corrosion.

3. High-Temperature Aging (550~650℃)

Reduced Strength:

Precipitated phases coarsen, dislocation bypass mechanism becomes dominant, resulting in decreased strength (e.g., 17-4PH after aging at 620℃ has an HRC of approximately 30~35).

Improved Toughness:

Coarse precipitates reduce stress concentration, significantly improving impact toughness, suitable for components requiring impact resistance (e.g., pump shafts, valves).

Optimal Corrosion Resistance:

Grain boundary precipitates dissolve at high temperatures, reducing intergranular corrosion sensitivity.

4.Over-Aging (Excessively High Aging Temperature or Excessively Long Aging Time)

If the aging temperature exceeds 650℃ or the holding time is too long, the precipitated phases will become excessively coarsened or even undergo secondary recrystallization, leading to a sharp decrease in strength and hardness. Simultaneously, grain growth may occur, further deteriorating mechanical properties.