Mechanical & microstructural evaluation of reversible and irreversible embrittlement in ultra-high strength steel

Journal Pre-proof Mechanical & microstructural evaluation of reversible and irreversible embrittlement in ultra-high strength steel Muhammad Samiuddin, Hira Younus, Zubia Anwer, Jinglong Li, Sumair Uddin Siddiqui, Mohammad Nouman Siddiqui PII:

S2588-8404(20)30017-2

DOI:

https://doi.org/10.1016/j.ijlmm.2020.02.003

Reference:

IJLMM 104

To appear in:

International Journal of Lightweight Materials and Manufacture

Received Date: 7 December 2019 Revised Date:

23 January 2020

Accepted Date: 5 February 2020

Please cite this article as: M. Samiuddin, H. Younus, Z. Anwer, J. Li, S.U. Siddiqui, M.N. Siddiqui, Mechanical & microstructural evaluation of reversible and irreversible embrittlement in ultra-high strength steel, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/ j.ijlmm.2020.02.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.

Manuscript title: Mechanical & microstructural evaluation of reversible and irreversible embrittlement in ultra-high strength steel Order of Authors: Muhammad Samiuddin*a,b,c , Hira Younusc, Zubia Anwerc , Jinglong Lia,b, Sumair Uddin Siddiquia,b, Mohammad Nouman Siddiquic Affiliations: aState Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China b

Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern

Polytechnical University, Xi’an 710072, PR China c

Metallurgical Engineering Department, NED University of engineering and

technology, Karachi, 75850, Pakistan Corresponding Author: Muhammad Samiuddin [email protected], [email protected], *Tel.: +86-13087500295, +923349551711

Co-Authors: [email protected], [email protected], [email protected], [email protected], [email protected]

ABSTRACT This comprehensive study gives evidence of reversible and irreversible embrittlement in ultrahigh-strength steel through destructive testing. Several tempering temperatures ranging from 350°C to 700°C were designated to observe temper embrittlement in the steel. Charpy impact, hardness , and tensile tests were performed for assessing the mechanical behavior of the steel. Stereo microscopy was utilized to examine the fractured surfaces and microstructural analysis was performed with the help of an optical microscope. During tempering, reversible temper embrittlement occurs due to the formation of alloy carbides between martensite plates while irreversible temper embrittlement is preferably due to the segregation of impurity elements (i.e. P and S) on grain boundaries as well as due to the coarsening of alloy carbides. It was found that irreversible temper embrittlement transpired when tempering was done in the range of 400450°C and reversible temper embrittlement occurred in 650-700°C temperature range. Results of the notched-bar impact test clearly showed a reduction in impact toughness in susceptible temperature ranges; moreover, the revealed fracture surface and micrograph also validate these findings. Experimental results also validate that both types of embrittlement are detrimental to the mechanical properties owing to the decrease in hardness and tensile strength. Keyword: Reversible & irreversible temper embrittlement, tempering temperature, impact strength, hardness, tensile strength.

1. INTRODUCTION Advanced high strength steels (AHSS) are materials of prominent interest for lightweight manufacturing and other structural applications. Lightweight materials are on the mainstream in aerospace, automotive industries, defense-related applications and in several mechanical components including gears, bearings, shafts and cam etc. [1-5]. In the establishment of lightweight automobiles, advanced high strength steels (AHSS) are considered as an attractive material owing to its high strength & good formability characteristics [3]. Among AHSS steels, Ni-Cr based alloy (e.g. AISI 4340) is the commonly used steel due to its high mechanical strength and good ductility. As per literature, tensile strength up to 2 GPa can be achieved through a well-designed heat treatment cycle. Such amalgamation of high strength and ductility made this alloy a strong candidate for several industrial applications [5]. Most of the researchers have put their efforts to improve the mechanical properties of AHSS steel by thermomechanical treatment. Maikranz-Valentin et al. applied localized heat treatment to form martensite after austenitizing, keeping the adjacent area non-martensitic to get the high plastic strain [6]. D.D munera et al. joined two different plates of steel (one being ductile & other brittle) through welding to acquire weight reduction [7]. Yunsong Xu et al. adopted a newly developed process to increase the mechanical properties of QP980 steel by quench and partitioning process to develop retained austenite along with the martensitic structure [8]. Microstructure plays an imperative role in determining tensile strength, yielding and ductility of the steel. Controlling the phases present in the steel is of prime importance to get the optimum mechanical properties. In practice, ferrite, pearlite, martensite and bainite are the typical phase constituents that are formed during heat treatment & other metalworking operations [9]. Among these phases, the presence of intermetallics and carbides are also responsible to ascribe different characteristics to the metal. Therefore, the mechanical properties of the steel can be further enhanced by utilizing an optimum heat treatment cycle [10]. However, imparted strength through heat treatment operations makes steels more prone to temper embrittlement and hydrogen embrittlement (HE). Thus, care should be taken in selecting temperatures for the hardening and tempering process. Past researchers showed that steel containing nickel as a major element or normally Ni-Cr steels have greater susceptibility towards temper embrittlement. Additionally, Cr-Mo steel is also prone to temper embrittlement [11]. Japan pressure vessel counsel conducted a research study on CrMo steel and the relationship of temper embrittlement and H2 embrittlement in Cr-Mo steels was

investigated. The results showed that the tendency to display H2 embrittlement was less severe in highly pressurized hydrogen vessels i.e. 15 MPa at 450°C [12]. Studies concerning the issues of synergistic effects on temper and H2 embrittlement are rarely encountered. Research conducted on temper embrittlement and hydrogen embrittlement of AHSS declared that atomic hydrogen enhances the grain boundary segregation of impurity element i.e. P which promotes temper embrittlement. An increase in the degree of hydrogen promotes both temper and hydrogen embrittlements in steel [13]. A similar work undertaken by Kaishu et al. showed that temper embrittlement in Cr-Mo steels is related to the concentration of impurities at prior austenitic grain boundaries which causes grain boundary embrittlement [14]. Abdollah et al. performed experiments on 32NiCrMoV12 steel to examine the temper embrittlement phenomenon. Results revealed that carbide precipitation and segregation of impurity elements lead to Temper Martensite Embrittlement (TME). Impurity elements such as P and S, initially, segregate into the grain boundaries during annealing which affluence the disintegration of the phase boundary between retained austenite and martensite. This results in the successive development of precipitate films that ascend during tempering. These brittle inter-lath carbides might provide crack nucleation sites which comprehend crack growth [15]. It is a well-established fact that through simple heat treatment, the combination of high strength, toughness and excellent fatigue resistance can be accomplished in AHSS steel [16, 17]. Moreover, it has been experimentally verified that the mechanical properties i.e. strength and toughness are equally incompatible [18] and the phenomenon of tempered martensite embrittlement (TME) is an obstacle in the development of high toughness steel [14]. The tempered martensite embrittlement is related to a rapid decline in room temperature Charpy impact toughness [20]. An occurrence of TME can be characterized by the inspection of fractured surfaces. A transgranular type fracture ensued in steel is due to the decomposition of retained austenite through coarse carbides along lath boundaries, while, an intergranular fracture is proceeded by simultaneous action of carbide precipitates & impurities at the prior austenite grain boundaries [19, 20]. Ni-Cr steel is generally heat-treated by oil quenching, proceeded by tempering. The advantage of tempering is that brittleness of martensite (structure with supersaturated carbon atoms) considerably reduces with the depletion of carbon atoms, as a result, martensitic toughness increases [21]. Mechanical properties of steel are solely dependent upon different phases that are

formed in response to heat treatment. These are necessary to grasp the high strength that can be unveiled by suitable heat treatment, in most cases, by quenching and tempering [21-22]. In this research work, high strength with medium carbon steel (see table-1) is selected to study the effect of tempering temperature to determine the embrittlement temperature (if occurred) caused due to the segregation of alloy carbides and other precipitates. Tempering range is selected from 350°C to 700°C (with +5 degrees to compensate temperature calibration) as most of the embrittlement occurred in this range. The critical temperature is governed by mechanical tests which include Charpy impact, tensile and hardness. A significant decrease in mechanical properties revealed the commencement of embrittlement which is well supported by fractured surfaces and microstructural examination. 2. EXPERIMENTAL WORK 2.1 Sample Preparation: In this study, wrought ultra-high-strength steel AISI 4340 was used. The dimensions of the square rod were 20” by 20” mm in cross-section and 3 ft in length. The chemical composition obtained with the help of Spectroscopy, according to ASTM (E 1085-09), is mentioned in Table 1: Table 1: weight percentages of constituent elements present in steel Elements Weight percentages

C

Si

Mn

P

S

Cr

Ni

Cu

Al

V

0.45

0.26

0.37

0.012

0.007

1.31

4.11

0.071

0.021

0.037

As received square rod was cut down into 10 samples for heat treatment and evaluation of mechanical properties (i.e. impact, tensile strength, and hardness) against the corresponding

microstructures. Impact toughness was determined according to Charpy standard (ASTM E23, ISO 148). Tensile samples were prepared in accordance with the ASTM E8 standard as shown in Figure 1. All the samples were made from an electric discharge machine (EDM) [29]. A notch making machine was used for engraving v-notch to impact specimens having a notch depth of 2 mm at an angle of 45o with a root radius of 0.25 mm as shown in Figure 1. Figure 1: Tensile and Charpy V-notch impact test specimens 2.2 Heat Treatment Cycles: All the samples were heat-treated according to the experimental scheme as presented in Figure 2 which involved normalizing, quenching and tempering at different temperature ranges. Normalizing was done to ascertain the uniformity in microstructure which is induced through the bulk deformation process i.e. forging. After normalizing, samples were re-heated to 800°C temperature and immediately quenched in an oil bath at room temperature to form a fully

martensitic structure. After quenching, the tempering process was performed from 350°C to 700°C with a variation of 50°C temperature to determine the reversible and irreversible temper embrittlement phenomenon.

Figure 2: Scheme of Experiments used in this research work 2.3 Microstructural and Mechanical Characterization: To reveal the microstructures all the samples were ground by standard metallographic techniques utilizing 120 to 2000 grit size emery papers sequentially. After grinding, polishing was done at 350 rpm disk speed in a clockwise direction using 0.5 micron alumina suspension with standard polishing cloths (Metkon, turkey) till a mirror-like surface was obtained. The chemical etching was performed with Villella's reagent having a composition of 5 ml HCl containing 1 gm picric acid and 100 ml of ethanol. Freshly prepared etchant was used to expose microstructural features. With that, Metkon Metallurgical Microscope IMM 901 was used for microstructure analysis. Hardness testing was performed by Rockwell hardness tester according to ASTM E18 standard. Three hardness readings were taken from each sample and an averaged value was considered for each specimen. Impact testing was performed after the heat treatment process according to

ASTM E23 and ISO 148 while tensile testing was performed in compliance with ASTM E8 standard using a universal tensile testing machine. 3 RESULTS & DISCUSSION All the samples were normalized at 870°C and then oil quenched after reheating at 800°C as per Figure 2. Tempering was implemented in a temperature range from 350°C to 700°C by utilizing a 50°C temperature difference. Table 2 presented the results obtained after performing mechanical characterization: Table 2: Test results of experimental work Heat treatment

Impact energy

Hardness

Tensile strength

temperatures(°C)

(Joules)

(HRC)

(MPa)

Normalized at 870°C

77.5

41.13

1162.72

Quenched at 800°C

17.5

42.83

1318.2

40

40.3

1078.2

405

42.5

32.66

915.98

455

32.5

39

966.68

505

37.5

26.16

954.85

555

62.5

31.83

966.68

605

77.5

25.3

915.98

655

87.5

21.3

763.88

705

45.5

28

855.14

355°C

3.1 INFLUENCE OF TEMPERING TEMPERATURE ON MECHANICAL PROPERTIES:

Impact strength, hardness, and tensile strength are the leading mechanical properties for any ultra-high-strength steel. Their significance cannot be underestimated. These mechanical properties have been determined as a function of tempering temperatures. Three samples were tested for each measurement. The impact strength, tensile strength, and hardness of as-received samples were measured without any heat treatment, whereas the other samples were heat treated

as normalized, quenched and the remaining samples were then tempered at different tempering temperatures as shown in Figure 2. Under the normalized condition, steel exhibits the highest impact energy and hardness value compared to the as-quenched sample as shown in (Figure 3a) and (Figure 3b) respectively. Since normalized sample contained pearlitic colonies with pro-eutectoid ferrite (light areas in Figure 4b), the toughness was enhanced whereas high hardness was obtained due to the formation of fine lamellae (dark regions in Figure 4b). The as-quenched sample has the lowest impact energy and high strength as shown in Figure 3c. The reason behind this can be explained on the basis of phase formation during the hardening of steel, where, on rapid cooling/quenching, the facecentered cubic structure of steel immediately transformed into a body-centered tetragonal (martensitic). At the same time, a large amount of distortion occurred due to the development of martensitic platelets which rapidly increases the tensile strength and lowers impact toughness. The martensitic structure has many dislocations and associated strains which reduces impact strength dramatically [23]. Figure 3 indicates that the impact toughness and hardness of working steel are much sensitive to tempering temperatures compared with the tensile strength. From Figure 3a, it can be seen that as the tempering temperature increases, impact strength rises significantly showing approximately a linear trend with the exception at extreme tempering temperatures which implies the onset of embrittlement. Tempering always accompanied microstructural changes that fetch variations in mechanical properties. Some prominent observations are notable during the tempering of the steel. Within the range of 400°C to 500°C, the impact strength found its lowest value with maximum hardness. The results showed an inverse relationship between impact strength and hardness. Consequently, it indicates the evidence of irreversible embrittlement, occurred at the tempering range of 400°C to 500°C which should be avoided if high impact toughness is required. This kind of behavior can be well supported with the following description: fine coherent and well-dispersed carbides within martensitic laths are beneficial to strength and impact properties [32, 33], but at high tempering temperature, coarse and non-coherent carbides are formed at martensitic lath boundaries. A large number of carbides are segregated at tempering temperature up to 500°C, a film of M23C6 carbides nucleate heterogeneously over phase boundaries of martensite laths, grain boundaries and across the boundaries of initial

austenite grains (IAG) as shown in Figures 4e and f. Presence of such coarse carbides are harmful to the impact properties as it weakens grain boundaries and provides easy path for crack propagation. In the tempering range between 450°C to 650°C, impact toughness increases progressively due to spherodization and dispersion of carbides with the depletion of carbon atoms from the martensitic structure. Thus, as a general rule, segregations of such coarse and non-coherent carbide films cause irreversible temper embrittleness that lowers impact toughness [27]. Reversible temper embrittlement also called temper embrittlement occurred at a temperature

between 650°C to 700°C. It can be noted that impact energy suddenly falls at 700°C with the evolution of deleterious precipitates which grows due to the presence of impurity elements. Apart from precipitation, grain coarsening is another combating factor that governed the mechanical properties of tempered steel. Several studies concluded that alloying elements like P, Si, and S increase hardness and decrease impact strength. At high temperatures, these alloying elements segregate along grain boundaries and cause a decline in impact strength [27]. Furthermore, other alloying elements such as aluminum, copper and vanadium (all present in working steel) causes embrittlement at high temperatures by forming carbides [28]. This type of embrittlement can be overcome by practicing following provisions: impurity elements that are responsible to temper embrittlement should be kept at lowest levels; rapid cooling from the susceptible tempering temperature to avoid the growth of deleterious precipitates; the addition of alloying elements e.g. Mo to suppress the precipitate formation and high-temperature thermomechanical treatment. Godbole et al. recently proposed a process to avoid temper embrittlement in martensitic steel. He increased the fraction of martensite by grain boundary engineering approach using Q&P heat treatment. This treatment increases the low energy lattice site by a transformation of retained austenite to martensite through tempering after the Q&P process which leads to reduced segregation of impurity elements and ultimately improved the toughness of 12Cr steel [32]. The variations in hardness may be due to the precipitation or dissolution of transition alloying elements at different tempering temperatures. It can be noted that the hardness of AISI 4340 steel exhibited inconsistent behavior; it decreased with the increase in tempering temperatures as shown in Figure 3b. It is a notable fact that variation in hardness with tempering temperature

depends upon the thickness of martensitic laths, grain size, distribution of fine precipitates, its density and the amount of carbon.

At higher tempering temperatures, growth of carbide

precipitates intensifies, martensitic laths are transformed to plate type with the increase of lath thickness (see Figures 4e and f) and a grain growth causes a reduction in hardness, as well as impact toughness [24-26]. Tensile strength exhibited a non-compliance propensity towards tempering temperature which can be confirmed from Figure 3c. Theoretically, the literature showed that there is very minute or no effect on tensile strength as the material undergoes irreversible and reversible embrittlement but the practical working shows some variations. A small dip in tensile strength has appeared in Figure 3c at high tempering temperature which has happened owing to grain growth and coarsening of carbides that can be evidently seen in Figures 4g and h. Additionally, the dislocation density also decreases that accounts for a decrease in tensile strength. Due to high tempering temperature, diffusion of carbon atoms from the martensitic structure accelerated which yielded a low fraction of martensite. Another possible reason for this variation in tensile strength is the instability of retained austenite that transforms into the islands of ferrite upon loading, thus inducing softness which ultimately increases its impact toughness at the expense of tensile strength [33].

Figure 3: Effect of tempering temperatures on the mechanical properties of treated samples: (a) Impact energy, (b) Hardness and (c) Tensile Strength. 3.2 INFLUENCE OF TEMPERING TEMPERATURE ON MICROSTRUCTURE: Figures 4a, b & c represent the microstructures of as received, normalized and quenched samples respectively. Effects of heat treatment on grain morphologies and phase distributions are clearly evident. Grains became homogenous after normalizing in contrast with the asreceived condition. The as-received structure consists of acicular ferrite-pearlite with finely dispersed martensite and bainite showing the effects of the thermomechanical process [30]. After normalizing, pro-eutectoid ferrite along the grain boundary regions (light color) with some pearlite colonies (dark color) have appeared as shown in (Figure 4b). As per literature,

such kind of microstructure persuades toughness in the material which is consistent with the experimental findings as shown in Table 2. Figure 4c shows the formation of martensite after quenching at 800 0C, retaining needles like characteristic appearance which is responsible for the increase in hardness. At 350°C temperature, martensitic laths grow relative to as quench structure along with some carbides precipitation. As the tempering temperature increases, the onset of lath carbides formation occurred within the martensitic matrix (Figure 4d). The carbide formation started from lath martensite and is segregated to initiate fracture (light regions). At further increase in tempering temperature, the structure contains retained austenite that transforms into carbides. Annihilation of as-quenched martensite occurred with the gain in carbide size which ultimately reduces the impact strength of the material (Figures 4e and f). A sudden fall in impact strength represents the phenomenon of temper martensite embrittlement (irreversible temper embrittlement). [27]

Above 450° C the impact strength rises possibly due to the dislocation annihilation phenomenon, and now the microstructure consists of a ferrite matrix with carbides, scattered throughout (Figure 4g). Temper embrittlement (reversible temper embrittlement) is observed at 700°C as the upsurge in impact strength once again declined which is due to the coarsening of grains, as evident in Figure 4h. [28] The influence of reversible and irreversible embrittlement is also manifested from the tensile behavior of the steel as shown in Figure 3c. Reduction in tensile strength at 400°C and 650°C tempering temperature indicates the occurrence of temper embrittlement. One interesting fact to note here is that the degree of reduction in strength is greater in reversible embrittlement as compared to irreversible embrittlement. The possible reason behind this trend can be

answered by analyzing their corresponding microstructures i.e. (Figure 4e) and (Figure 4h) respectively. Since grain size and distribution of second phases play a significant role in determining the mechanical behavior of a material, thus, microstructure tempered at 400°C is appeared to be finer than that of the sample treated at 650°C tempering temperature. Consequently, the irreversible embrittled sample displayed high strength. In addition to this, phase distribution and coarsening of martensite along with carbide depletion have heightened this behavior. Tempering between embrittlement ranges did not cause any critical change in

the tensile strength of AISI 4340 due to some transition alloying elements as discussed earlier [28]. In general, a declining trend is observed which is solely due to grain coarsening and stress reliving owing to the martensite dissolution.

Figure 4: Microstructures of heat-treated samples (a) as received (b) Normalized (c) asquenched (d) tempered at 350°C (e) tempered at 400°C (f) tempered at 450°C (g) tempered at 650°C (h) tempered at 700°C 3.3 EFFECT OF TEMPERING TEMPERATURE ON FRACTURED SURFACES: (Figures 5a to g) show stereo macrographs of heat-treated samples revealing their characteristic fracture surfaces. Figure 5a depicts the fracture surface of the normalized sample and displays rough and dull surface near top-notch position with some chevron markings that represent the direction of crack growth by forming fan-like ridges. Asquenched sample has a shiny fracture surface which reveals that it fractured in a transgranular fashion, indicating a complete brittle fracture due to martensitic structure, as revealed in Figure 5b. Remaining 5 samples treated at different tempering temperatures depicts discontinuous fracture paths except for the one treated at 650°C (i.e. Figure 5f). Figures 5c, d and e show mixed mode of fracture, in a way that at the edges, all the samples are fractured in a brittle fashion which transformed to ductile at the middle section recognized by its fibrous appearance. Absence of chevron markings specifies the presence of temper martensite embrittlement (irreversible temper embrittlement) at 450°C. Plus, the

presence of some shallow dimples in this structure evident a reduction in toughness at this temperature. Moreover, the formation of irregular dimples assured the presence of brittle carbides that produces uneven and discontinuous fracture path [28]. Figure 5f indicates the fractured surface of the sample tempered at 650°C. Fracture appearance is quite similar to the normalized sample. Some shallow ridges are formed during the fracture. Such a dramatic change in fracture appearance can be correlated with the intuition developed in the previous section that described the reason for the rise in impact toughness. At 700°C tempering temperature, fracture appearance again changes and continuous fracture path indicates the presence of temper embrittlement (reversible temper embrittlement). At this temperature, elongated and deep dimples are produced due to intense grain coarsening and carbide precipitation. De-cohesion occurred with the rupture of atomic bonds weakened at high temperatures. The influence of segregating alloying elements and some hardening elements (discussed earlier) played a key role in the de-cohesion of atomic bonds. All these factors lead to continuous brittle intergranular fracture [31], as shown in Figure 5g.

Figure 5: Stereo images of fractured surfaces at different heat-treated conditions (a) Normalized (b) as-quenched (c) tempered at 350°C (d) tempered at 400°C (e) tempered at 450°C (f) tempered at 650°C (g) tempered at 700°C 4. CONCLUSION: 1. The study focused on the microstructural changes by performing heat treatment at different tempering temperatures. Results showed that different tempering temperatures bring changes in the microstructure which ultimately leads to different mechanical properties (include impact strength, hardness, and tensile strength). 2. The selected high strength steel displayed two different types of temper embrittlement which occurred at 400°C and 700°C tempering temperatures known as irreversible and reversible temper embrittlement respectively. A rapid decline in impact toughness and tensile strength

at the above-mentioned temperatures corroborates the existence of embrittlement in such type of high strength steel. Therefore, care should be taken while carrying out heat treatment specially tempering. One should avoid the two extremities of tempering temperature while practicing heat treatment. 3. Formation of carbide precipitate in between martensite laths and along the austenite grain boundaries are responsible for dwindling in impact toughness which occurred at 450°C and is termed as irreversible tempered embrittlement. Impurity elements (P and S) along with some other hardening elements (Al, Si, V) also causes embrittlement through grain boundary segregation; this type of phenomenon is known as reversible temper embrittlement. 4. Change of fracture appearance (see Figures 5a & f) indicated the presence of temper martensite embrittlement (irreversible temper embrittlement) occurred at 450°C while

continuous fracture path shows the presence of temper embrittlement (reversible temper embrittlement) transpired at 700°C.

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