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Precipitation Hardening Stainless Steels

Introduction

Stainless steel is the name given to a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium. Just as there is a range of structural and engineering carbon steels meeting different requirements of strength, weldability and toughness, so there is a wide range of stainless steels with progressively higher levels of corrosion resistance and strength. This results from the controlled addition of alloying elements, each offering specific attributes in respect of strength and ability to resist different environments. The available grades of stainless steel can be classified into five basic families: ferritic, martensitic, austenitic, duplex and precipitation hardening.

Precipitation Hardening Stainless Steels

Precipitation hardened, PH,  stainless steels are a class of stainless steels that can be hardened to significant strength levels by special heat treatments. These alloys were first introduced in the mid 1940s to fulfil the requirement for high strength corrosion resistant alloys with higher toughness than the plain martensitic stainless steels, (and increased tempering-off/softening resistance when exposed to slightly elevated temperatures). Many different alloys have been developed over the  subsequent years, and they are now widely used in aerospace, marine, automotive, and other specialist applications. These alloys are used whenever a particular combination of high strength, corrosion resistance, and toughness is required that  no other type of stainless steel can provide. Precipitation hardening is typically achieved by the addition of elements such as copper, molybdenum, niobium, aluminium, and titanium in various combinations and levels. After heat treatment they can give tensile strengths ranging from 850 MPa. to 1700 MPa. and yield strengths ranging from 520 MPa.  to over 1500 MPa. Thus they are about three to four times stronger than the common austenitic stainless steels such as Type 304L, BS EN 1.4307, and Type 316L, BS EN 1.4404.

The family of precipitation hardening stainless steels is comprised of three main sub-groups, which describes their matrix structure, i.e. Martensitic, Semi-Austenitic and Austenitic. They are united by the fact that they use a precipitation/ageing mechanism to achieve their hardness. One can look at these stainless steels as thus containing alloying elements  which control their final matrix structure, (i.e. mainly the chromium and nickel balance), and other alloying elements, (as already indicated), which form precipitates in that matrix and harden it. In some cases, these age hardening elements, as they precipitate out and/or cluster during the pre-precipitation stage, can also alter the stability of the matrix phases, and thus the final matrix phase balance.

  • Martensitic Precipitation Hardening Stainless Steels

Historically, these were the first PH steels to be developed, and a typical example of this group is 17-4 PH, BS EN 1.4542, UNS S17400. As can be seen in the summary table below, the “17-4” refers to the steel’s approximate chromium and nickel contents, respectively. For the precipitation/age hardening effect the steel is alloyed with the elements copper and niobium. During the hardening cycle it transforms to martensite at low temperatures, typically around 250oC, and is further strengthened by ageing at about 482oC. The most common product form is bar, but there is some availability as castings, sheet, or plate, particularly in the USA, where these steels were first developed. Cold forming of these alloys is difficult because of the hard, untempered martensitic structure developed after the solution heat treatment step. Alloys in this condition have relative low ductility and high strength. Hardening by a single aging treatment will produce tensile strengths typically from 790 MPa to 1310 MPa. This particular alloy can be used at temperatures up to  around 470oC, i.e. below the ageing temperature. Where the original ageing reaction is produced at higher temperatures in other steels in this sub-group, the temperatures encountered in its application can be correspondingly higher.

As shipped from the mill the material will usually be in the solution heat treated condition, (Condition A), thus ready for fabrication and finally hardening by the customer. However, it can be supplied hardened, or in an overaged condition for forging/cold heading at the customer’s request.

Thus the possible as-supplied conditions are as follows:-

    • Condition A, (Solution Treated/Annealed), used when fabrication and precipitation heat treatment will be carried out by the user. However, if severe cold forming is needed then Condition H 1150 or H 1150-M is suggested
    • Condition H 1075 Precipitation hardened condition, machinability is similar to that of Condition A.
    • Condition H 1150 Precipitation hardened condition. Fabrication is easier than material in Condition A. Providing subsequent deformation is not significant, no further heat treatment steps are necessary.
    • Condition H 1150-M This has a softer martensite matrix thus improving the steel’s machinability.
    • Condition H 1150 + H1150 This is a heat treatment meeting NACE MR01750/ISO 1516 and NACE MR0103, it is sometimes referred to as H 1150-D, where D means Double precipitation hardening

 

  • Semi-Austenitic Precipitation Hardening Stainless Steels

As one might expect from their name, this sub-group has a mixed matrix microstructure consisting of austenite and martensite. The typical example of this group is 17-7 PH, BS EN 1.4568, UNS S17700, and again its commonly used name, “17-7”,  indicates its chromium and nickel contents, respectively. The alloying element which is added to give the age hardening effect in this particular alloy is aluminium, as can be seen in the table below. As supplied from the mill normally this grade will have been solution treated to Condition ‘A’, by heating to approximately 1066oC, (1950oF), and holding for up to 4 hours. User then fabricate the component required from the condition ‘A’ material. At this point there are three main routes that the processing can follow:-

    • Route 1. This is where the steel has been heavily cold worked during fabrication , (this is called Condition ‘C’) . The steel is then heated to 482oC, (900oF), held for at at least 1 hour then air cooled. The material is now in condition CH 900. It is generally acknowledged that this gives the highest aged strength levels, as per AMS 5529.
    • Route 2. The first step is the austenite conditioning step where the steel is heated to 955oC, (1750oF), held for approximately 10 minutes then air cooled, the material is now said to be in condition ‘A1750’.
      Within a maximum of 1 hour the steel is cooled to -73oC, (-100oF), and held at that temperature for 8 hours, after which it is allowed to warm in air back to room temperature. The material is now said to be in condition ‘R100’. This is called the transformation step.
      In the final stage the steel is heated to 510oC, (950oF), held for 90 minutes then air cooled to room temperature. The steel is now said to be in condition ‘RH950’. This is the precipitation hardening/ageing step and results in a material with intermediate final strength levels.
    • Route 3. Again, the first step is austenite conditioning, but the steel is heated to  a lower temperature of 760oC, (1400oF), held for approximately 90 minutes, then then within 1 hour  cooled to 13oC, (55oF), held for at least 30 minutes. The material is now said to be in condition ‘T’.  This latter step is again called the transformation step.
      In the final step the steel is heated to 565oC, (1050oF), held for 90 minutes, then air cooled to room temperature. The steel is now said to be in condition ‘TH1050’. This is precipitation hardening/ageing step and results in the lowest final aged strength levels for this alloy.

This somewhat complicated list of possible processing options is typical of this subgroup of Precipitation Hardening Stainless Steels.
As rapid cooling from the mill solution treatment temperature to room temperature retains a fully austenitic structure, this provides these steels with the necessary ductility for cold-forming processes, unlike the martensitic PH steels which tend to be excessively hard at the same stage. To induce hardening and strengthening, an initial transformation from austenite to martensite is necessary. This prepares the material for subsequent treatment at the aging temperature. Heating semi-austenitic PH steels to the range of 650oC–870°C prompts the precipitation of carbides. As alluded to above, this lowers the level of the alloying elements in the matrix and in particular those that stabilise austenite, thus allowing a degree of transformation to martensite upon subsequent cooling to room temperature. Partial martensite transformation of the austenite can also be achieved by refrigeration below the Ms temperature, (the beginning of martensite transformation), or through cold working, (the Md temperature being higher than the Ms temperature).

The other effect that is apparent with this sub-group is that because the solubility of the alloying elements increases at higher temperatures, the martensite start, Ms and finish, Mf temperatures can be controlled by the solution heat-treatment temperatures. At high solution temperatures, the alloy content of austenite is increased, and the martensite start temperature is depressed. At lower solution temperatures, the austenite is leaner in alloy content, (i.e. lower levels are in solution), and upon cooling transforms to martensite.

Whatever the route the final step is always an ageing step which hardens the steel. The hardening does not come primarily from dispersion strengthening, but rather because the precipitates exhibit  varying degrees of coherency of their crystal lattice with that of the parent structure. There is a mismatch however between the two lattices and this produces strain in the parent lattice, and thus a hardening effect. The material can be overaged and in this stage the precipitates start to loose coherency with the matrix and their hardening effect diminishes. As well as the kinetics of ageing, the ageing temperature will affect the number of precipitates per unit volume, with a more numerous finer dispersion resulting from lower ageing temperature, but precipitation will be slower.

The best availability of sheet and strip products in the USA is for grades PH 15-7 Mo and AM-350, whereas in the UK 17-7 PH and FV520, BS EN 1.4594, UNS S45000, have the best availability. Grade 15-7 PH is similar to the 17-7 PH alloy, but has a low molybdenum content, (see table below), that provides even higher strength levels in the age hardening process. AM-350 is similar to both 15-7PH and 17-7 PH, but has a slightly better high-temperature capability. FV520B, employs molybdenum, copper and niobium as the precipitate forming elements, and is a “14-5” PH steel, as can be seen in the summary table below.

For some of these alloys a refrigeration step is always necessary, (e.g. -50oC/-60oC for eight hours), in order for transformation to a stable austenitic/martensitic structure, although the two most commonly used alloys, FV520 and 17/7PH, do not require refrigeration to develop optimum* properties. (*However, optimum in one application may not necessarily be the optimum combination of properties for another application.)

  • Austenitic Precipitation Hardening Stainless Steels

These alloy have a fully austenitic matrix, so their hardness comes from the precipitates that form on ageing. The most common alloys in this subgroup is A286, sources also cite 17-10 P, but evidently this is not a popular alloy with consequently very limited availability. The former is alloyed with molybdenum, aluminium, titanium, vanadium and boron for precipitation purposes, and would be  referred to as “15-26” PH, using the system used for referring to the other PH alloys. In 17-10 P, phosphorus is the precipitate forming element, see summary table below, which is suggested to adversely affect its weldability.
Austenitic alloys maintain their austenitic structures through annealing and subsequent hardening via aging. At the annealing temperature of 1095oC –1120oC, the precipitation hardening phase dissolves and remains in solution during rapid cooling.

Ageing of these alloys occurs at temperatures between 500oC to 760oC. The austenitic grades are stable down to room temperature, improvements in strength being from the precipitates formed by ageing at 650oC to 750oC. These fully austenitic grades can exhibit good toughness and some may be used at cryogenic temperatures.

The strength and the hardness of austenitic alloys is lower than that of martensitic or semi-austenitic PH grades, and they retain their non-magnetic properties.

Nominal Chemical Composition of Selected PH Stainless Steels

Alloy UNS No. Typical Composition, %
C Mn Si Cr Ni Mo Cu Ti Other
Martensitic
PH 13-8Mo S13800 0.05 0.10 0.10 12.8 8.0 2.3 Al=1.1
15-5 PH S15500 0.07 1.0 1.0 14.8 4.5 3.5 Nb=0.3
17-4 PH S17400 0.09 1.0 1.0 16.3 4.0 4.0 Nb=0.3
Custom 455 S45500 0.05 0.5 0.5 12.0 8.5 0.5 2.0 1.1 Nb=0.3
 
Semi-austenitic
15-7Mo PH S15700 0.09 1.0 1.0 15.0 7.1 2.5 Al=1.1
17-7 PH S17700 0.08 0.9 0.5 16.5 7.5 Al=1.0
AM-350 S35000 0.09 0.8 0.3 16.5 4.3 2.75 N=0.10
FV 520 S45000 0.05 0.6 14.5 4.75 1.4 1.7 Nb=0.3
Sandvik Nanoflex S46910 <0.012 12.0 9.0 4.0 2.0 0.9 Al=0.35
 
Austenitic
A-286 S66286 0.08 2.0 1.0 15.0 25.5 1.25 Ti=2.1

Al</=0.35

V=0.3

17-10P PH 0.07 0.75 17.2 10.8 P=0.28

 

 

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