Challenges and opportunities in the additive manufacturing of stainless steels. Part 4 – Precipitation hardenable martensitic stainless steels

Stainless Steel World is proud to present a series of technical articles on additive manufacturing written by renowned expert Professor Todd Palmer from Pennsylvania State University. In Part 4 he considers the additive manufacturing of precipitation hardenable martensitic stainless steels.

By T.A. Palmer (a,b, a) Department of Materials Science and Engineering and (b) Department of  Engineering Science and Mechanics, The Pennsylvania State University, USA

Precipitation Hardenable (PH) martensitic grade stainless steels display strength levels that far exceed those observed in austenitic, ferritic, and duplex grades, making them an attractive material for applications across the aerospace, marine, and power generation industries. Unlike these other stainless steels, PH grades require the application of controlled post-process heat treatments to achieve the specified property levels. The combination of these high strengths and excellent weldability makes PH grade stainless steels a popular choice for use in fusion-based additive manufacturing processes. However, significant variations in as-deposited microstructures, heat treatment response, and mechanical properties of additively manufactured components have been widely observed. Much of this variability originates from the use of powder feedstocks which introduce oxygen and nitrogen levels much higher than those present in their wrought counterparts. Even though these elements are not typically monitored, their presence in additively manufactured materials drives the formation of selected phases at much higher levels. For example, high oxygen levels originating in the powders lead to the formation of oxide inclusions in the as-deposited and post-processed materials. When high nitrogen levels are combined with the presence of other austenite stabilizers, such as nickel and manganese, high levels of retained austenite can form and the martensite start temperature can be significant. With the interest in using these alloys in additive manufacturing, an improved understanding of the role of composition on solidification and microstructural evolution, particularly during post-process heat treatment, will need to be addressed. Building a solid knowledge base in these areas will be critical to producing well-controlled structures and properties for critical applications where additive manufacturing will be employed.

What makes PH stainless steels unique?

Precipitation Hardened (PH) martensitic grade stainless steels differ in both structure and properties from the more commonly used austenitic and duplex grades. While these other grades are used where corrosion resistance is the primary selection criteria, martensitic stainless steels are the material of choice when strength in corrosive environments is a more important consideration. With design strengths on the order of 1520 MPa (220 ksi), PH grade stainless steels are used in applications spanning the aerospace, marine, chemical, food processing, power generation, and paper industries.
In the wrought condition, these high strengths are obtained through a martensitic microstructure combined with the formation of small nano-sized precipitates formed during well-controlled aging heat treatments. The most common PH martensitic grades are 17-4 PH (UNS 17400) and 15-5 PH (UNS 1550), with compositions summarized in Table 11. There are rather minimal differences between the two since the 15-5 PH grade was designed as an updated version of the 17-4 PH grade to achieve improved properties. Both alloys are dominated by similar nickel and chromium levels, which provide corrosion resistance and the ability to easily form martensite. Age hardening is obtained by adding copper, in the order of 3.0 to 5.0 wt.%, leading to the precipitation of nano-sized copper precipitates during low-temperature heat treatments.
A well-controlled two-step heat treatment process consisting of a high temperature solutionizing and a low temperature aging step is specified and used to obtain a range of mechanical properties. The high-temperature solutionizing heat treatment is performed at approximately 1040 ºC and is designed to produce a homogenous distribution of alloying elements and a single-phase austenitic microstructure. Martensite will then form when the material is cooled. Since these alloys are not particularly quench sensitive, a martensitic structure can form at cooling rates consistent with air cooling.
After this intermediate solutionizing heat treatment, the materials are subjected to a low temperature aging heat treatment at temperatures between approximately 495 ºC and 650 ºC. These heat treatments are designed to produce small uniformly distributed nanometer-sized copper precipitates in the martensitic microstructure. Peak aging conditions, or those defined as having the highest hardness or strength, are obtained at the lower aging temperatures. As the aging temperatures increase, the copper precipitates coarsen and the resulting hardness and strength decrease, producing an overaged microstructure. Even with the lower mechanical properties, there are benefits to using these steels in the overaged condition, particularly when improved corrosion properties are desired.

What happens when PH stainless steels are additively manufactured?

The additive manufacturing (AM) of PH grade stainless steels is commonplace, with nearly all commercial powder bed fusion (PBF), binder jetting, and directed energy deposition system manufacturers offering 17-4 PH grade stainless steel as a material option2. Much of the popularity of the alloy for AM processing can be traced to its wide availability in powder form, owing to its common use in metal injection molding (MIM) and other powder metallurgy (P/M) processes. In addition to the ease with which the alloy powder can be obtained, 17-4 PH grade stainless steel is viewed as having excellent weldability and does not typically display solidification cracking or other common fusion welding defects when AM processed. This second characteristic, in particular, has made it a popular alloy for fabricating test parts and prototypes.

Much of the current focus on the AM of these alloys is on the use of powder bed fusion (PBF) processes. Moving from prototypes to large volume production runs, particularly for components fabricated using PBF AM processes, represents a much bigger challenge for the AM processing of PH grade stainless steels. These challenges become apparent when characterizing these materials in the as-deposited condition, where they have exhibited wide variability in microstructure and properties. Much of this variation has been connected to the seemingly unpredictable appearance of different levels of retained austenite, which can range from nearly 0% to upwards of 97% in an alloy that is nominally a martensitic grade stainless steel3,4. Such large changes in the amount of retained austenite visibly affect both the magnetic response, with high levels of retained austenite making the material non-magnetic, and the resulting mechanical properties. In materials with high levels of retained austenite, the engineering stress-strain curves display a discontinuous yielding phenomenon similar to that observed in transformation induced plasticity (TRIP) steels. In this case, the retained austenite undergoes a strain-induced transformation to martensite when subjected to the highly controlled stresses and low strain rates used in conventional mechanical testing. Such large variations in retained austenite levels also lead to significantly different heat treatment responses, with materials containing little to no retained austenite aging as expected when using standard heat treatment practices. At high retained austenite levels, however, the materials do not age in the same manner and require higher temperatures to reach peak aging, if they display any aging effect at all.
Such wide differences in properties arise primarily from the impact of minor alloying elements in the powders being used as the feedstocks for the AM processes. In the wrought condition, the oxygen and nitrogen levels are not typically monitored since they can be maintained at low levels through controlled steel making practices and thermo-mechanical processing. Powders, on the other hand, are susceptible to both oxygen and nitrogen pick-up during melting and the subsequent gas atomization process. In the fabrication of the powders, both argon and nitrogen are used during the melting process. As a result, they will display much higher levels of oxygen and nitrogen than the corresponding wrought materials. The presence of such high levels of oxygen and nitrogen can have different effects. High oxygen levels, which can reach compositions on the order of 0.04 wt.% in the powder, can lead to the formation of oxide inclusions both within the powder particles as well as in the as-deposited materials5. Similar conditions have been observed in both austenitic and duplex stainless steels, but as with those stainless steel grades, the composition has a significant impact on the type and structure of the resulting inclusions. Previous investigations of austenitic and duplex stainless steels have identified the evolution of manganese and chromium-rich phases that originate in the powders and as-deposited materials as amorphous particles5. After HIP post-processing, the inclusions transformed into the predicted phases, such as the appearance of spinel in super duplex stainless steels. In PH grade stainless steels, these inclusions differ in both composition and structure and have been found to be rich in silicon and amorphous in structure, and remain that way even after the application of post-process heat treatments.
On the other hand, nitrogen levels can vary quite widely, from low levels of 0.01 wt.% to high levels approaching 0.14 wt.%. Such significant differences in nitrogen composition are a major factor in the large variations in retained austenite, mechanical properties, and aging response of different heats of material. In most cases, these differences in nitrogen are attributed to the selection of the atomization gas, with materials with low nitrogen levels typically being atomized with argon and those with higher nitrogen levels being atomized using nitrogen gas. While the selection of atomization gas has been shown to impact the flow properties of austenitic stainless steels6, melting practices and the atmosphere used during electric arc melting have a much greater impact and need to be monitored.
Since the impact of these high nitrogen levels is not well understood, both industry and the standards organizations require the use of low nitrogen powders7. This tightened specification for powder limits the nitrogen levels to less than 0.03 wt.%, as shown in Table 1, and brings the heat treatment response more in line with that observed in wrought materials. However, high nitrogen levels might not be negative and may contribute to the development of alloys with significantly improved mechanical and corrosion properties. The key to their use will be in determining the appropriate heat treatment processes to achieve what can be considered peak aged conditions. In materials with nitrogen compositions of 0.14 wt.%, significant improvements in the mechanical properties over low nitrogen materials in both the as-deposited and aged conditions have been observed4. Both low and high nitrogen materials display strengths and elongations equivalent or greater than wrought minimums. However, the mechanical properties in the high nitrogen materials far exceeded those observed in the low nitrogen materials, typically on the order of 10% and more.

Path forward

As with austenitic and duplex stainless steels, the AM processing of PH grade martensitic stainless steels produces both structures and properties far different from more familiar wrought conditions. PH grade stainless steels, however, present additional challenges since they require well-controlled post-process heat treatments to achieve their specified properties. With the complex thermal histories produced by the layer-by-layer build methodology characteristic of AM processing, the as-deposited microstructures will be far different than those in wrought conditions.

While PH grade stainless steels are an attractive alloy for AM processing, there are several challenges to their more widespread implementation in critical applications. Foremost among these challenges is the minimization of the large variability in properties driven by changes in composition. Heat-to-heat variability in the heat treatment response of both 15-5 PH and 17-4 PH grades arising from small changes in nitrogen or other critical alloying elements need be overcome. This challenge will eventually highlight the importance of tracking composition and defining acceptable composition ranges. Improving the understanding of the evolution of these microstructures, particularly with variations in nitrogen compositions, will be necessary to expand the use of AM processing for these alloys, particularly in critical applications.
It is important to remember that, especially under AM and rapid solidification conditions, even common stainless steel alloys like the 17-4 PH grade should be considered complex multi-component alloys.
New approaches based on both in situ and ex situ characterization and computation modeling will be needed to capture the complexity of the solidification and solid-state transformations in PH grade stainless steels,. Advanced tools capable of monitoring these transformations over these small spatial and temporal scales are now available to researchers. For example, high-resolution transmission electron microscopy provides atomic-scale resolution, and synchrotron-based x-ray techniques capture phase transformations at microsecond level time resolutions using in situ techniques. These tools become more powerful when coupled with computational thermodynamics and numerical modeling. As these tools are coupled with fundamental metallurgical principles, researchers and academics can begin to expand the understanding of the impact of these processing conditions on these complex alloys and transfer them to industrial practice.