Effect of shot peening on corrosion resistance of additive manufactured 17-4PH steel

Steel 17-4PH (UNS S17400, AISI 630, 1.4542) has found applications in the chemical, aerospace, energy, marine, and biomedical sectors [1]. It is one of the frequently used steels for precipitation hardening and fabrication of injection molds [2]. The group of precipitation hardening steels was created as a result of the demand associated with World War II for inexpensive materials with high strength at elevated temperatures [3]. The strengthening mechanism is based on precipitation of highly dispersed copper particles in the martensitic phase [4]. Martensitic steel has a predominantly austenitic matrix at solutionizing temperatures of approximately 1,040–1,065°C, while the process of cooling to room temperature triggers a transformation that turns it into a martensitic phase [5].

Shot peening (SP) is a common treatment used to increase the strength and fatigue life of metal components [6]. The strengthening occurs by introducing compressive residual stresses in the near-surface layer and increasing the dislocation density [7]. With SP, hardness, roughness, and corrosion resistance can be modified [8]. SP can increase fatigue resistance, resistance to fretting, rolling contact fatigue, or resistance to stress corrosion cracking [9].

SP is based on imparting particles with a spherical or near-spherical shape with sufficient velocity so that they are able to plastically deform the surfaces of the component being modified. By deforming the material, residual compressive stresses are generated in the material, and strengthening by crushing and fragmentation of grains within the surface layer takes place. A schematic representation of the changes within the surface layer is shown in Figure 1. As can be seen in Figure 1, SP can close cracks formed in the manufacturing process [10]. Shots can be made, for example, from steel, glass, ceramics, or nutshells [11]. Particle acceleration is accomplished typically using compressed clean air. During the flight, the particles collide with each other as well as with the surface of the component being processed [12].

Fig. 1

Additive manufacturing (AM) is an innovative manufacturing method that converts a three-dimensional (3D) CAD digital model into a physical model [14]. Manufacturing involves dividing the model into slices and subsequently fabricating layer by layer [15]. However, additively manufactured steel is not free of discontinuities, pores, sometimes unmelted particles, and so on, which can directly affect corrosion resistance. In addition, the direction of fabrication can affect porosity [16]. Therefore, other treatments are often used to improve these properties [17]. Another characteristic is the lower density of the material, as studies have confirmed that SP is an effective treatment for increasing the hardness of the material, particularly when combined with precipitation hardening [18].

For 17-4PH steels, precipitation hardening, that is, solution treatment combined with aging, is often used. These treatments are aimed at dissolving the intermetallic phase and increasing hardness. In 17-4PH steel, there are usually two phases, martensite and austenite, and the austenite can be either primary or reversed [19]. In some studies, ferrite was also detected after AM.

The structure of 17-4 PH steel may contain both austenite and martensite. It is worth noting here that SP can lead to strain-induced transformation of austenite to martensite [20]. Both indirect and direct transformations have been observed. However, the most common transformation is the γ-ε-α^′ transformation. Above that, SP around improves stress-corrosion, corrosion, and fatigue properties. However, it can be a detrimental process when high cyclic stresses are present [21]. The biggest impact on the phase composition is due to the manufacturing technology, as well as the material or gases used (for atomization and as a shielding gas) [22]. According to Starr et al. [23], the use of nitrogen atomized powder in an argon atmosphere had little effect, as the material was more than 96% austenitic. However, the same (nitrogen atomized) powder when produced in a nitrogen atmosphere resulted in an austenite content of 99.3%. Using powder atomized in argon and fabricating in nitrogen resulted in 76% martensite content. Using both atomization and fabrication in an argon atmosphere, 87% martensite was obtained. Rafi et al. [24] also confirmed this relationship regarding phase composition. In addition, the hardness was examined, in the as-fabricated state; the highest hardness was found in steel from powder atomized in nitrogen and produced with an argon atmosphere around 280 HV. The lowest hardness of around 230 HV was obtained in the nitrogen–nitrogen regime.

17-4PH steel has relatively good corrosion resistance comparable to duplex (austenitic–ferritic) steels. Guo et al. [25] compared the resistance of AISI 304 steel with 17-4PH; however its mechanical properties and hardness were found to be higher. Most of the information that can be found on 17-4PH steel is similar and describes the resistance as similar to 304 in most environments; in some, however, it shows slightly better properties. However, the resistance of the steel after AM may differ from materials produced by classical manufacturing methods. Thus, the corrosion properties of steel can be affected by the conditions of manufacturing and processing. Stoudt et al. [26] noted the hazards that can occur in environments in which chloride and halide ions are present. Despite the good resistance of this group of steels in aerated aqueous environments, localized attacks can occur. The corrosion resistance of stainless steels comes from a thermodynamically stable Cr-rich passivating oxide layer. Steel 17-4PH has the best corrosion resistance in dilute aqueous solutions in the group of hardened stainless steel alloys [27].

Lara Banda et al. [28] studied the passivation of 17-4PH steel in citric acid. They found that by passivating in citric acid, it was possible to produce a passivation-resistant layer in the presence of sodium chloride (NaCl) and sulfuric acid H₂SO₄. Karaminezhaad et al. [29] conducted a study on the effect of molybdenum on stress corrosion cracking for 17-4PH steel. They found that I_c and I_pass decrease with increasing Mo content. Moreover, it was found that the extent of passivation increases with an increase in molybdenum content. Moreover, its addition does not facilitate the formation of an oxide layer and increases resistance to pitting corrosion. Tavares et al. [19] conducted a research on the effect of sulfur on the corrosion resistance of 17-4PH steel. Two samples with very low and acceptable sulfur content after different heat treatments were investigated. The degree of sensitization (DOS) was higher for samples in the high-sulfur H900 condition due to preferential attack on MnS inclusions. Intergranular corrosion was also detected in the H1150 state due to the precipitation of chromium carbides and the formation of reverse austenite. H900 condition indicates aging at 900°F for 1 hr and in H1150 it indicates aging in 1,150°F for about 4 hr. In addition, all high-sulfur samples had lower pitting potentials (E_pit) than low-sulfur samples. 17-4PH steel is often used in gas and oil production systems, and acidic environments such as H₂S, CO₂, and/or water are present [30]. 17-4PH steel is subject to sulfide stress cracking. In this type of cracking, environmental factors such as temperature, molecular pressure of H₂S, pH of the environment, or the presence of chlorides have the greatest influence.

Nakhaie and Moayed [31] studied the resistance of 17-4PH steel to pitting corrosion after cold rolling. They found no significant effect on the potential for pitting. However, cold rolling was found to increase the dissolution rate of metastable pitting in a way that facilitated the transition from metastable to stable pitting. Raja and Rao [32] studied the behavior of welded joints, and found that better corrosion resistance was exhibited by steel after solution treatment and aging at the highest temperature than by solutionized steel. In the case of steel after the welding process, pitting was found at the boundary of d-ferrite and martensite and in the heat affected zone at the grain boundary. Kosasang et al. [33] investigated the effect of aging on the corrosion resistance of 17-4PH steel in artificial salvia. Pitting corrosion occurred where pores were already present. The mechanism was determined to be a two-stage process; in the first stage, dense corrosion products are formed followed by accelerated dissolution by peeling. The highest resistance was obtained with aging at 480°C in both electrochemical measurements and the immersion test. Barroux et al. [34] studied the pitting corrosion resistance of 17-4PH steel fabricated by laser beam melting (LBM) technique and found that the pitting initiation points were lack-of-fusion pores. However, they did not affect the overall corrosion resistance due to their small number and when considering i_pass and E_pit, the LBM-manufactured steel exhibited better properties than the wrought material. Differences in the behavior of individual pits in additively manufactured versus wrought material were also indicated. Metastable pits formed on LBM were very few, characterized by long lifetimes and large amounts of charge, while wrought presented more metastable pits with short lifetimes and small amounts of charge.

Corrosion resistance can be affected by the scanning strategy in AM in powder bed fusion processes [35]. The use of concentric scanning can improve corrosion resistance by up to about 30%. The use of double scanning or hexagonal strategy did not result in changes in corrosion resistance. However, the impact is not fully investigated and further research should be conducted in this field.

SP-induced changes in the surface layer, as well as changes in the microstructure, can potentially affect the corrosion resistance of steel. During cold processing, a transformation of metastable austenite into martensite and intermediate carbides like Fe₃C called “Martensitic stress-induced transformation” (MIST) can occur [36], which can affect corrosion resistance. One of the dangers during SP is the potential penetration of burnishing medium particles into surfaces, thus reducing or increasing corrosion resistance. It was also found that treatment in solution followed by peak aging increased the resistance of the weld and the material within the heat affected zone [32].

Since AM is a completely different method from conventional ones, the 17-4PH grade is characterized by a complexity of structures depending on the implemented process. As far as the authors' knowledge goes, there are no studies depicting the SP of 17-4PH steel with glass beads often used in the manufacture of injection molds as well as describing the corrosion behavior of 17-4PH steel after the SP process. This paper attempts to answer the question of how the corrosion resistance of 17-4PH steel changes after SP, as well as how its operating properties such as hardness and roughness change. This work aims to assess how corrosion resistance may be affected by particles locked in surface, and which penning parameters are most appropriate, that is, particle size and pressure.

The test object was X5CrNiCuNb16-4 (17-4PH) steel samples printed from GP1 metal powder manufactured by EOS (see Figure 2B). The chemical composition of the test specimens is shown in Table 2. The 3D printing process was carried out in direct metal laser sintering (DMLS) technology. EOSINT M280 device was used (from EOS GmbH) using optimal printing parameters from EOS′ closed software license, among others: laser power was 200 W, sintering thickness was 0.02 mm, and laser spot size was 0.1 mm. The particle of GP1 powder that was used was of the order of 10–50 μm on average. Powder particles were characterized by mostly spherical shape with some satellites. Nitrogen was used as a shielding gas for fabrication.

Fig. 2

Four cylindrical test specimens were made and cleaned with compressed air and by ultrasonic cleaning. The specimens of 17-4PH steel were in the shape of disks with dimensions of ∅30 mm and H = 6 mm. The appearance of the samples and GP1 powder is shown in Figure 2. The chemical composition of the tested steel is given in Table 1.

The external surfaces of the specimens in the X–Y horizontal plane were subjected to SP (perpendicular to the surface) using Lepco's Peenmatic micro 750S. Three different sphere-like media were used for SP: ceramic beads, glass balls, and CrNi steel shot. A peening pressure of 0.3 MPa was applied and the surface treatment time was 60 s. The distance from the nozzle to the face of the specimen's surface was constant at 20 mm. The characteristics of the modified specimens and the designations adopted in the work are given in Table 1.

Analysis of the chemical composition showed a composition in line with the requirements outlined in the standards ASTM A564, EN10088-1, as well as the manufacturer's declarations.

SP increased the hardness of the surface layer of 17-4PH steel produced by the DMLS technique, which combined with reduced roughness should translate into improved fatigue strength. After SP using glass beads, the highest hardness was obtained right beneath the surface at around 459 HV_0.2, with a slightly lower hardness obtained after using steel shots at 450 HV_0.2 and ceramics at 436 HV_0.2. The use of steel shots enabled to increase the hardness in the deepest part in the material. This is because the increase in hardness depends on the value of compressive residual stresses. The value of the compressive residual stress deep in the material depends on the energy of a single shot; a constant peening pressure therefore results from its mass.

The surface hardness was 241, 496, 522, and 501 HV_0.2 for Ref., ceramic, glass, and steel, respectively. It is worth noting here that structure defects such as pores, unmelted powder particles, inclusions or particles remaining after the SP process can directly affect the value of surface hardness.

A 7% and 12% reduction in Ra roughness after SP was achieved for ceramics and steel, respectively, demonstrating that SP can be an effective post-processing method after 3D printing. SP glass beads resulted in a 10% increase in roughness.

Three phases, γ-austenite, α-martensite, and ferrite, were detected in the as-fabricated condition. After peening two phases, γ-austenite and α-martensite, were detected in all of the samples. The size of the crystallites was smallest after processing with glass, 15 nm slightly larger sizes were obtained after using ceramics at 16 nm. The largest size of crystallites was obtained for Ref. After peening, the largest size of crystallites was characterized by the sample processed using steel shots.

The steel texture was typical for additively manufactured steel. Longitudinal notches caused by fragmentation of the medium are visible, as well as indentations from spherical particles, especially in the case of the glass peened sample. The surface topography changed after peening, and the strengthening in the near-surface layer reached 40, 60, and 80 μm for ceramic, glass, and steel, respectively.

EDS analysis confirmed the presence of residual particles used for SP in the structure, which can change material properties after SP.

The highest performance in terms of corrosion resistance after SP was obtained consecutively for steel, glass, and ceramics. Roughness had the greatest effect on corrosion resistance; lower roughness influences more favorable electrochemical parameters. The effect of crystallite size was also noted for larger crystallites in the structure after SP had a favorable impact on corrosion performance. The tests confirmed the good mechanical properties of 17-4PH steel combined with relatively high corrosion resistance.