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One grade, different properties
17-4 PH names an alloy; H900 describes its ageing condition. Two products can meet the same grade chemistry while having different hardness and strength. This is one reason PH steels are versatile: different conditions within an alloy family meet different mechanical needs.
PH stands for precipitation hardening. Fine particles formed within the material during ageing increase strength. For the user, the effect appears in hardness, resistance to permanent deformation and ductility. Condition names and tables help explain these differences without requiring detailed metallurgy.
What do Condition A and H codes mean?
Condition A is the solution-treated starting state. Its properties differ from those of aged material. The number in H900, H1025 or H1150 identifies the nominal ageing temperature in Fahrenheit. H900 therefore means approximately 482 °C, not 900 °C.
| Designation | Meaning | Key distinction |
|---|---|---|
| Condition A | Solution-treated condition | Not the same as the final aged condition |
| H900 / H1025 / H1150 | Ageing conditions named after Fahrenheit temperatures | Time and starting condition depend on grade |
| H1150M | A special two-temperature condition | Not the same as single-stage H1150 |
| H1150 + H1150 | Two ageing cycles with cooling between them | Not treated as identical to one 8 h hold |
| TH / RH / CH | Conditions also identifying the preparation route | Particularly relevant to 17-7 PH and 15-7 Mo |
| +P930 / +P1070 | Strength classes in EN designations | The number is not an ageing temperature |
17-4 PH and 15-5 PH: temperature and time table
Common ageing temperatures and times follow the same sequence for these two grades. This does not mean every mechanical property or product specification is identical. The main ageing conditions following Condition A are listed below.
| Condition | Temperature (°C) | Time at temperature | Cooling |
|---|---|---|---|
| H900 | 482 | 1 h | Air |
| H925 | 496 | 4 h | Air |
| H1025 | 551 | 4 h | Air |
| H1075 | 580 | 4 h | Air |
| H1100 | 593 | 4 h | Air |
| H1150 | 621 | 4 h | Air |
| H1150 + H1150 | 621 → 621 | 4 h → 4 h | Air after each stage |
| H1150M | 760 → 621 | 2 h → 4 h | Air after each stage |
Is higher hardness always better?
Hardness measures resistance to indentation. Yield strength marks the stress where permanent deformation begins, while tensile strength is the maximum stress reached in a tensile test. Elongation and toughness describe other aspects of deformation and energy absorption. These properties are related but do not measure the same thing.
For 17-4 PH and 15-5 PH, higher-temperature ageing than H900 generally reduces hardness and strength while ductility and toughness can increase. H1150 is therefore not a lower-quality version of H900; it provides a different property balance. Suitability depends on the combination of properties the component needs.
The same H code is not the same cycle in every grade
Reading a condition code separately from its grade can be misleading. H900 specifies a 1 h hold for 17-4 PH but a 4 h hold for Custom 455 at the same nominal temperature. The common 13-8 Mo PH series begins at H950. Custom 465 also has distinct cold-treatment preparation and final cooling requirements.
| Grade | Example conditions | Distinguishing point |
|---|---|---|
| 13-8 Mo PH | H950, H1000, H1025, H1050, H1100, H1150, H1150M | 4 h for the basic ageing conditions |
| Custom 455 | H900, H950, H1000, H1050 | 4 h including H900 |
| Custom 465 | H950–H1150, H1150M | A/CT start; time and cooling depend on product condition |
| 1.4530 / 1.4596 | Conditions identified by temperature or strength class | Not automatically equated to H codes |
Why are 17-7 PH, 15-7 Mo and AM 355 different?
For 17-7 PH and 15-7 Mo, the state before final ageing is part of the condition designation. TH1050 and RH950 use different routes from Condition A; CH900 follows cold-rolled Condition C. They should not be read merely as three different furnace temperatures.
AM 355 has SCT conditions involving subzero cooling and tempering, as well as a double-ageing route. The grade tables distinguish these routes so that one PH steel’s treatment table is not applied to the whole family.
How should the table values be read?
Heat-treatment temperature is not service temperature. The 621 °C listed for H1150 describes ageing, not continuous operation at that temperature. Holding time likewise means time at the specified material temperature, not total time from switching on the furnace.
In mechanical-property tables, typical, minimum and maximum mean different things. Typical results illustrate material behaviour; acceptance limits depend on the product specification. Transverse sheet results and longitudinal bar results cannot be used interchangeably without considering section and test conditions. Grade pages therefore identify product form, test direction and value type.
These tables help explain conditions and material properties. The actual heat-treatment cycle and acceptance criteria for a particular component are established by the governing specification and material documentation.