Introduction
There has always been a need for stronger, harder, more corrosion resistant and generally more versatile metal alloys. The properties, described above are in high demand in nuclear, aerospace, food processing, chemical and metalworking industries [1]. Some examples of specific demands and application requirements for these industries include the making of critical peripheral structural components and fuel handling assembly for nuclear plants [2], exposure to highly corrosive marine environments on marine vessels [3], bulk digesters for paper manufacturing [3], turbine blades design [3] and pipes for oil rigs [3].
Luckily, there is a material that can tick all of the boxes for the applications stated above, and that material is PH (precipitation hardening) stainless steel. It provides high yield strength from 515 to 1415 MPa, high tensile strength from 860 to 1520 MPa and ability to elongate between 1 and 25%, depending on the prior treatment, alongside with great corrosion resistance [4].
Additionally, its mechanical properties are greatly dependant on the heat treatment, meaning that the manufacturer can easily alter them if he/she desires to do so [5], especially, since the material can be supplied in a ‘solution treated’ condition, that allows it be machined straight away [7].
Characterisation
Precipitation hardening stainless steels are a set of stainless steels (iron-chromium-nickel alloys) with additions of titanium, aluminium, niobium, molybdenum and copper, which give them higher strength than average stainless steels [5]. They can be classified into three main groups according to their martensite finish and start temperatures and the final properties and behaviour: martensitic, semi-austenitic and austenitic, as can be seen from table 1 [5]. Table 1 also shows us some of the alloys that exist within each group, with the most common ones being 17-4PH for martensitic, 17-7PH for semi-austenitic and A-286 for austenitic type [6].
17-4PH stainless steel is considered to be the most common precipitation hardening stainless steel [6]. It consists of a dual-phase microstructure with δ ferrite and martensite [12]. It contains ~17% chromium, which gives it very good corrosion resistance properties, ~4% nickel, ~4% copper and ~0.3% niobium, which account for its excellent mechanical properties up to 589K [8]. Depending on a specified precipitation and annealing processes (varying the temperature and the length of the process), a range of desired properties can be obtained [Table 2] [5].
Table 1. Compositions of precipitation hardening stainless steels [5]
The highest tensile strength of 1310 MPa with the maximum yield strength of 1170 MPa and the maximum hardness of 48 [Table 2] can be obtained due to precipitation hardening. The highest elongation that can be obtained is 18% [Table 2] [5]. As stated above, the material is widely used in many different industries with applications such as engine parts, bearings, gears, boat fittings, valves, pump shafts, etc [9].
17-7PH stainless steel is a semi-austenitic precipitation hardening stainless steel with a composition of ~17% chromium, ~7% nickel and ~1% aluminium. It has maximum possible tensile strength of 2515 MPa, maximum possible yield strength of 1310 MPa, 7% elongation and hardness of 46 HRC [Table 3] [5]. Due to these properties, the material is widely used in the airspace industry, since it can sustain high temperatures under high loads. It is also important to note that the described material is not suitable for use in oxidizing environments and salt water, as it can undergo corrosion. Therefore, it should only be used in oxidising and mild chemical environments, as well as in fresh water [10].
Table 2. Minimum mechanical properties of precipitation-hardening martensitic type stainless steels per specification noted [5] (legend in table 4)
Table 3. (same columns as table 2) Minimum mechanical properties of precipitation-hardening semi-austenitic type stainless steels per specification noted [5] (legend in table 4)
A-286 is an austenitic precipitation hardening stainless steel with a composition of ~15% chromium, ~25% nickel, ~1.3% molybdenum, ~2% titanium, maximum of 0.35% aluminium, ~0.3% vanadium and 0.003% boron [Table 1] [5]. It has maximum tensile strength of 965 MPa, yield strength of 655 MPa and elongation of up to 25% [Table 4] [5]. It can operate at temperatures up to 977K, meaning that it is a great material for applications that require high temperature tolerance, alongside with corrosion resistance and high strength. For example, such applications include jet engine blades manufacturing, castings, afterburner parts and super-charger components. It can also operate at sub-zero temperatures if required [11].
Table 4. (same columns as table 2) Minimum mechanical properties of precipitation-hardening austenitic type stainless steels per specification noted [5] (+ legend)
Processing
17-4PH stainless steel is an age-hardening martensitic alloy, which turns completely to martensite at around 523K [6] during air cooling, since it has the martensite finish temperature just above the room temperature [5]. The steel is enforced by copper particles precipitation in the martensite matrix prior to the cooling process [13]. The steel is then hardened during an ageing process for a short time of 1 to 4 hours at a temperature between 753K and 893K [5]. During the ageing process the steel forms sub-microscopic, face-centred cubic copper-rich phase as can be seen in table 5 [5] [13].
Table 5. Precipitation-hardening phases in precipitation-hardening steels [5]. 17-4PH, 17-7PH and A-286 phases can be observed in the table.
17-7PH has a martensite start temperature greatly below the room temperature, meaning that the alloy transforms mainly to the austenitic phase during cooling. That makes the material very ductile in that condition [5]. In order to form the martensite phase, carbon (and other alloying elements) precipitation takes place, which both strengthens the material and increases martensite start and martensite finish temperatures. In order to raise the martensite start temperature to a room temperature, low conditioning temperature between 1003K and 1033K is used, which allows the martensite phase to form during the cooling process. However, it is also possible to use high conditioning temperature between 1203K and 1228K, allowing less carbon to precipitate, meaning martensite finish temperature would remain below zero. That means that refrigeration would be required in order for the martensite phase to form. The advantage of using higher temperature lies in the higher final strength of the material due to a higher carbon content. Subsequently, cold working may also be used to achieve transformation. Finally, ageing at a temperature between 728K and 838K is used in order to harden the alloy [5].
A-286 austenitic alloy never transforms to the martensitic phase, because its martensitic start phase is too low. Therefore, the desired strengthening effect is obtained via the precipitation of various intermetallic compounds inside of the austenitic matrix. It must be noted that the best strengthening effect occurs at the aging temperatures between 728K and 783K, since higher temperatures decrease the material’s preparation time and strength, but increase its toughness and ductility, as can be seen on figure 1 [5].
Figure 1. Aging curves for selected precipitation-hardening stainless steels. (a) Aged at 753K. (b) Aged 783K. Arrows indicate standard ageing time for each steel [5].
Testing
The application-critical properties for precipitation hardening stainless steels consist of corrosion resistance, hardness, sliding wear, strength, toughness and ductility.
In order to measure corrosion resistance of precipitation hardening stainless steels, the samples should first undergo the preparation process using plasma nitriding with a DC plasma nitriding unit after a surface grounding process. Then X-ray diffraction analysis can be used to identify phases with metallography to characterise layer morphology [14].
Subsequently, hardness of the specimen can be measured using Vickers microhardness tester [14].
Also, the corrosion properties can be measured using an electrochemical testing technique with NaCl solution. That way the anodic polarisation curves are measured [14].
Additionally, in order to analyse sliding wear and corrosion-wear, pin-on-disk tribometer, calculating the wear volume and reciprocation, corrosion-wear tester in NaCl solution could be used respectively [14].
In order to measure tensile strength, yield strength and ductility, tensile testing could be performed using servo-hydraulic test machine with mechanical wedge grips, subsequently plotting a tensile-stress strain curve and analysing it [15] [16].
Possible Setbacks
There may be a number of possible types of material degradation, that may eventually result in failure. For example, if ageing of 17-4PH stainless steel occurs at a higher than suggested temperature large amounts of martensite would transform to austenite along martensite lath boundaries and incoherent precipitates would form in the matrix. That would result in depletion of large amounts of copper and nickel from the matrix, which would cause the enhanced electrochemical dissolution of the martensite matrix [17].
Also, SCC may occur in other types of precipitation hardening steel due to hydrogen embrittlement if the material is used in geothermal power applications in the presence of H_2 S in the surroundings. Hydrogen embrittlement may also occur in acidic NaCl solution [17].
Finally, there is always a danger of a mechanical failure due to an excessively large stress, that is applied to a material, especially if a material has low carbon content (for example 17-7PH stainless steel aged at low conditioning temperature) [5].
Overview and Analysis
It is clear that precipitation hardening stainless steels have a large number of applications in the modern world, ranging from airspace to marine to household, due to a large number of various alloys of different precipitation compositions that provide each metal with a unique property and characteristic. Hence, the manufacturing processes differ for each alloy composition slightly, even though they share a general similarity. Overall, the manufacturing process is relatively easy, meaning that the material can be widely used.
The material’s unique combination of high strength and corrosion resistance makes it extremely attractive for various engineering applications that were not possible prior to the creation of the material.
However, it must be noted that there are some limitations present in precipitation hardening stainless steels, such as possible stress corrosion cracking due to hydrogen embrittlement or a mechanical failure due to high stress. Therefore, it is clear that the material’ properties have to be checked first, before it can be used for a specific application.
Therefore, it is clear that in order to capitalise on the material’s strengths and advantages and avoid material’s limitations, precipitation hardening stainless steel should be used in the form of composites either merging with different types of precipitation hardening stainless steels or using polymer matrix. That way, if the design of a composite is correct, the material would be able to use PH stainless steel’s strength and corrosion resistant properties together with the ductile and lightweight properties of a polymer matrix increasing both the performance for existing applications and enlarging versatility for use in other yet unseen applications. Specifically, for composite making, stainless steel could be transformed using one of the grain orientation techniques in order to increase the composite’s strength in one direction. Also, products like wires made out of PH stainless steel could be used for the desired application.
Conclusion
Precipitation hardening stainless steels are a group of extremely useful materials with a wide variety of applications due to their unique combination of mechanical and corrosion resistive properties. The relatively easy manufacturing process of precipitation hardening stainless steels and the ability to change their properties by slightly altering their composition either via precipitation or via ageing process conditions makes them very popular in various industries. Even though there is a number of setbacks present, generally PH stainless steels are extremely useful and well-performing materials.
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