Microstructure Evaluation of Cryogenically Hardened and Tempered AISI H13 Hot Work Tool Steel

1. Introduction

Subzero treatment is a ecofriendly heat treating process [1,2] which is implemented in a rage of temperatures approximately from -72⁰C (use of dry ice), -185⁰C (use of liquid nitrogen) and ultimately to -268⁰C (use liquid helium) [3]. These treatments can be applied and positively affect materials made of different chemical bonding corresponding to metals (steels, Al, Cu, Mg alloys), carbides, polymers and metallic glasses [4,5]. Based on these temperatures, in case of steels, it can be grossly categorized as a) cold treatment when the treating temperature reaching a down limit of -80⁰C, b) shallow cryogenic treatment which is labeled when the treatment temperatures are in the range of -80⁰C to -160⁰C and c) the deep cryogenic treatment extending down to -196⁰C [6,7,8,9,10,11,12]. The process is based on microstructure alterations due to applied subzero temperatures resulted to substantially increase of the population and distribution of transition carbides forming at low tempering temperatures [13].

The metallurgical transformation occurred during cryotreatment stage is strongly dependent on the subzero temperature selected as well as on the holding time at that temperature. A hybrid model seems to adequately describe the precipitation mechanism of the transition carbides during low temperature tempering. The debated findings of carbon clustering before ε-carbide exerted by one dimensional atom probe tomography by Taylor et al [14] were verified by using three dimensional atom probe by Zhu et al. [15]. On the other hand, the classical model of the heterogeneous nucleation of ε-carbides is also supported by different researchers [16,17,18]. Analyzing and synthesizing researchers’ findings, carbon clustering (spinodal-like or segregation controlled) produces precursors which are followed by nucleation and growth of ε-carbides. The aforementioned mechanism takes advantage of subzero treatment, so the density and distribution of transition carbides is enhanced. Increased ε,η-carbides precipitation along with the austenite-to-martensite transformation in subzero temperatures is likely to be the main causes of the improved properties of alloyed steels after cryogenic treatment.

In case of tool steel grades, their response to the cryogenic treatment is varying as it depends on their chemical composition especially in carbon and alloying elements additions (Cr, Mo, V, W, Co) as well as on their heat treatment history [19]. According to weight percentage of carbon and of alloying elements, strength [6,20], toughness [21,22], wear [23] and corrosion resistance [24,25] are enhanced [26] respectively. A plot of temperature as function of time for a typical deep cryogenic treatment cycle is demonstrated in Figure 1 [3]. Subzero treatment is an additional process which can be applied in different stages of the heat treatment sequence, namely, between quenching and tempering, between tempering stages or after tempering. Moreover, it can be implemented in more than one cycle, sequential or not [26]. Evidently, in order cryogenic treatments to be effective they need to be carried out after martensitic quenching and before tempering [1,27,28,29].

Table 1. Examples of tool life improvement using cryotreatment [30,31,32,33].

Tooling	Initial Average working life	Average working life after cryotreatment
5-cm end mils used to cut C1065 steel	65 parts	200 parts
Hacksaw blades used to cut bosses on M107 shells	4h	6h
Zone punches used on shell casings	64 shells	5820 shells
Nosing thread dies used in metal working	225 shells	487 shells
Copper resistance welding tips	2 weeks	6 weeks
Progressive dies used in metal working	40.000 hits	250.000 hits
Blanking of heat treated AISI 4140 and 1095 steel	1.000 pieces	2.000 pieces
Broach used on a C1020 steel torque tube	1.810 parts	8.602 parts
Broaching operation on forged connecting rods	1.500 parts	8.600 parts
Gang milling T-nuts constructed from C1018 steel	3 bars	14 bars
with M2 cutters AMT-38 cut-off blades	60h	928h

Table 1. Examples of tool life improvement using cryotreatment [30,31,32,33].

Tooling	Initial Average working life	Average working life after cryotreatment
5-cm end mils used to cut C1065 steel	65 parts	200 parts
Hacksaw blades used to cut bosses on M107 shells	4h	6h
Zone punches used on shell casings	64 shells	5820 shells
Nosing thread dies used in metal working	225 shells	487 shells
Copper resistance welding tips	2 weeks	6 weeks
Progressive dies used in metal working	40.000 hits	250.000 hits
Blanking of heat treated AISI 4140 and 1095 steel	1.000 pieces	2.000 pieces
Broach used on a C1020 steel torque tube	1.810 parts	8.602 parts
Broaching operation on forged connecting rods	1.500 parts	8.600 parts
Gang milling T-nuts constructed from C1018 steel	3 bars	14 bars
with M2 cutters AMT-38 cut-off blades	60h	928h

In a more practical manner, the working life improvement of engineering components constructed of tool steel grades is given hereunder:

In all the above cases of tooling, a heat treatment sequence was implemented, comprising of quenching, subzero treatment and tempering stages. The early research, especially after 2005 and actual industry usage have proven the validity of the cryogenic process, research now is turning to determine why the results are seen and how to maximize those results [34]. Until now, there are many ambiguities in parameters like austenitizing temperature, quenching temperature, rate of cooling, temperature and holding period of subzero treatment, rate of warming-up, tempering temperatures and tempering period. There is need of further investigations so to optimize the parameters of deep cryogenic treatment process for different materials [35]. The improvement in mechanical properties by cryogenic processing is directly related to the combined effect of hardened and tempered martensitic matrix and carbides which are distributed in the microstructure. The austenite-to-martensite transformation, the starting temperature of carbide nucleation during warming up and the tempering sequence regulates the final microstructure of the material. In tool steel grades, the mix of carbides present in the tempered martensite matrix after the cryogenic treatment sequence are critical for their hardness, toughness and the wear resistance respectively [35,36,37]. The formation mechanism of carbides depend on the tempering behavior of each steel grade and the specific effect must be taken into account when designing their heat treatment sequence tailoring the properties of products [38].

Differences in tempering behavior after cryogenic heat treatment sequence of medium to high alloyed tool steel grades e.g. the temperature of secondary hardening peak has been reported [6,39,40,41,42]. The tempering diagrams of tool steel grades should be updated including their behavior after cryogenic treatment so the benefits of the specific treatment to be obtained constantly [43,44].

The tempering diagram of AISI H13 tool steel in the range of 180⁰C to 630⁰C has been constructed for different holding times and ramp up rates in shallow and deep cryogenic temperatures [45]. A comparative tempering diagram of H13 tool steel grade is demonstrated in Figure 2. During tempering of tool steels, there can be six overlapping stages of carbides evolution as temperature increases [41,46,47]:

Stage 0: carbon clustering starting from subzero temperatures to 150⁰C,

Stage 1: precipitation of transition carbides, extended between 150⁰C to 230⁰C,

Stage 2: austenite decomposition, mainly occurring between 230⁰C to 280⁰C presenting critical overlapping on both previous and followed stage,

Stage 3: in situ transformation of transition carbides to cementite, 260⁰C to 360⁰C,

Stage 4: heterogenous nucleation of alloy carbides provoking secondary hardening, 450⁰C-550⁰C,

Stage 5: coarsening of precipitated carbides, 550⁰C-650⁰C.

Tempering the tool steel over 350⁰C approximately is accompanied by a gradual decrease in hardness and tensile strength due to the loss of tetragonality of martensite and the escape of carbon from solid solution to form cementite. At higher tempering temperatures, a further decrease of hardness is caused by the coarsening of cementite and recrystallization of ferrite.

According to various researchers [48,49,50,51,52,53] in case of hardening H13 tool steel, primary carbides M₇C₃, M₆C and MC are expected to be present after austenitizing and quenching. Cryotreating and low tempering lead to in-situ nucleation of M₃C carbides on ε/η transition carbides. During secondary hardening, M₃C carbides transform to Cr rich M₇C₃ and Mo rich M₂C carbides respectively. Moreover, prolonged or repeated tempering lead M₇C₃ and M₂C carbides to transform to more stable carbides M₂₃C₆ and M₆C respectively.

In all the cases above, the austenitizing temperature was set to 1040⁰C and the soaking time was 30 min. All specimens were air cooled, and snap tempered to 100⁰C for 1 hour. Then the actual tempering time was 2h. In shallow cryogenic treatment, the temperature was held to -130⁰C for 30 min and the warming up duration was 30h (inside the cryogenic chamber). Moreover, in case of deep cryogenic treatment, the temperature was hold to -196⁰C for 24h and the warming up duration was 1h respectively. Differences between the conventional and cryogenic tempering diagrams have been reported e.g. the secondary hardness peak appeared in lower tempering temperature [39,40] or the secondary hardening been suppressed due to DCT [13,20]. Evidently, holding time to subzero temperature affects the tempering behavior of the tool steel in the range of secondary hardness [54].

The microstructural features of cryogenically treated and tempered H13 tool steel have been extensively demonstrated by various researchers using different heat treatment parameters as a) the austenitizing and tempering temperatures, b) the temperature and holding time of the subzero treatment, c) the position of the subzero treatment related to quenching and tempering based on the aim of their study [48,49,50,55,56,57,58]. Consequently, the documentation of the microstructure demonstrated by different researchers has been extracted for different heat treatment parameters without following specific patterns. The scope of the specific labor is to examine the evolution of the microstructure features acquired after shallow and deep cryogenic treatment under the same conditions so the results to be reproducible. Material of the same melt as well as heat treatment equipment e.g. for austenitizing, quenching, subzero treatment and tempering were used. Different tempering temperatures were chosen so the evolution of the microstructure to be demonstrated. Its features were documented via optical and scanning electron microscopy. A comparison of the microstructure of cryogenically shallow and deep cryogenic treated specimens covering critical tempering temperatures in the range of 180⁰C to 630⁰C is discussed.

2. Materials and Methods

2.1. Material and Treatment

A chromium-molybdenum-vanadium-alloyed having medium carbon %wt steel grade was used belonging to hot work tool steels family. AISI H13 has an exceptional combination of mechanical properties,namely, high level of toughness and ductility, uniform and high level of machinability and polishability, good resistance to abrasion at both low and high temperatures, good high-temperature strength and resistance to thermal fatigue, excellent through-hardening properties and very limited distortion during hardening [59]. The cast analysis of the plate used for the construction of the specimens is given in Table 2.

Forty specimens of dimensions 15x15x40mm divided in two equal batches, A (shallow cryogenic treatment) and B (deep cryogenic treatment). In sequential time periods were heated in a muffle furnace at a rate of 25◦C/min approximately to austenitizing temperature of 1040◦C. The soaking time for both treatments was the same, equal to 30 min. The specimens were wrapped with distilled paper and placed in a steel box so to prevent decarburization in case of direct contact with the atmosphere of the furnace. In both cases, the specimens were quenched via pressurized air. Immediately after quenching, all samples were snap tempered to 100◦C for 1h [60]. The specimens of batch A were cryotreated to -130◦C, holding time of 20min, having ramp up duration of 30h (physically warmed to ambient temperature inside the cryogenic chamber) as the specimens of batch B were cryotreated to -196◦C for 24h respectively using liquid nitrogen via cryogenic chamber (Cryotron LTI 40). The cooling rate was relatively slow to 2,5◦C/min.

A couple of specimens was used in each one of the ten temperatures selected so the evolution of microstructure and hardness during the range of 180◦C to 630◦C to be studied. The temperatures used for the specific study were 180⁰C, 280⁰C, 350⁰C, 450⁰C, 500⁰C, 525⁰C, 550⁰C, 575⁰C, 600⁰C, 630⁰C respectively covering all sequential stages of tool steels tempering. The duration of each tempering stage was 2h. In the present study, the evolution of the microstructure after low tempering to 180⁰C, as well as to the vicinity of secondary hardening temperature of the specific tool steel grade,namely, 500⁰C, 525⁰C and 550⁰C is implemented.

2.2. Method

One of the two specimens of each heat treatment sequence (austenitized, quenched cryotreated, snap tempered and tempered) was prepared for microhardness testing [61] and the other for metallographic analysis respectively [62]. Four specimens of each of the two subzero treatmets which were tempered at 180⁰C, 500⁰C, 525⁰C, 550⁰C, were cut, cold mounted and grinded using sequal grinding papers starting form P#80 to P#2000 grit size. Then, they were polished with alumina paste and emulsion of 6, 3, 1 and 0,3μm grain size sequentially. The microstructure analysis was held after etching the specimens with 5% Nital. Optical microscope Zeiss Axio Vert A1 as well as scanning electron microscopes (Jeol JSM-6390, Jeol JSM-7610F, Hitachi TM3030Plus) were utilized to depict the microstructural evolution of the cryotreated material.

3. Results and Discussion

3.1. Low Tempering

Optical microscopy of specimens subjected to deep (-196⁰C, hold for 24h, ramp up duration 1h, Figure 3a,b) and shallow (-130⁰C, hold for 0,3h, ramp up duration 30h, Figure 3c,d) cryotreatement, and low tempering to 180⁰C was employed. The microstructure of both subzero treatments is comprised of matrix of tempered lath martensite (Figure 3a, area I, Figure 3c, area III) [63] having acicular areas due to low tempering temperature, of dispersed carbides (Figure 3b, Figure 3d area III)) as well as of retained austenite (Figure 3b area II) [64].

In case of the deep cryotreated treatment a more detailed depiction of the features of lath martensite was elaborated using scanning electron microscope verifying the results from optical microscopy (Figure 4).Very fine dispersed sub-micron carbides in the matrix of tempered martensite are depicted [49]. Based on their size and the heat treatment sequence implemented transition (Figure 4a, area I)[65] and primary carbides are present (Figure 4a, area II) [24,26,66]. An element mapping via energy-dispersive X-ray spectroscopy was elaborated on the same specimen. Primary V, Cr and Mo rich carbides of were detected (Figure 4b).

Moreover, detailed observation on both microstructures via scanning electron microscope revealed carbide colonies located to the grain boundaries and globular sub-micron carbides precipitated inside the grains after shallow (Figure 5a) and deep cryogenic treatment respectively (Figure 5b) [26].

3.2. Tempering at 500⁰C

On both tempering diagrams constructed for shallow (-130⁰C/0,3h) and cryogenic (-196⁰C/24h) treatment of the specific tool steel grade (Figure 2) it presents secondary hardening peak.

The microstructure extracted by optical microscope of both deep (Figure 6a,b) and shallow (Figure 6c,d) cryotreated and tempered at 500⁰C samples, it is comprized of tempered martensite (Figure 6a,c) and dispersed carbides (Figure 6b,d). There is no clear evidence of the presence of retained austenite so its weight percent is expected to be less than 5%.

In case of deep cryogenic treatment, the matrix of tempered martensite has more distinct grain boundaries related to its morphology after shallow cryogenic treatment. Moreover, carbide precipitation is more pronounced on the grain boundaries after deep cryogenic treatment while in case of shallow cryogenic treatment, carbides are homogenically dispersed inside the matrix of the tempered martensite.

In case of deep cryogenic treatment and tempering at 500⁰C, the grain boundaries of tempered martensite were examined further. Extensive precipitation of secondary carbides was observed (Figure 7).

Using scanning electron microscope, sub-micron carbides are precipitated between the laths of tempered martensite in both subzero treatments (Figure 8) [67]. Through the elaborated microstructures, it is evident that the precipitation of interlath carbides is more intense in case of shallow cryotreatment and tempering at secondary hardening peak temperature related to deep cryotreatment respectively. The specific findings are verified by other researchers [56].

3.3. Tempering at 525⁰C

Through light optical microscopy, the main feature that can observed to the microstructure after tempering at 525⁰C is the formation of cementite within the tempered martensite phase [68,69] which, by comparison, is more homogenous after deep cryogenic treatment (Figure 9b,d).

Carbide precipitation between martensitic laths is noted via scanning electron microscope in both subzero treatments which were tempered at 525⁰C (Figure 10, Figure 11a). Moreover, distinct carbides precipitation is evident on the grain boundaries of shallow cryogenically treated not etched specimen (Figure 11b).

3.4. Tempering at 550⁰C

Extensive cementite formation as well as coarsening of the precipitated carbides are the main features of the microstructure on both cryotreatments after tempering at 550⁰C via optical microscopy (Figure 12). In case of shallow cryogenic treatment, the formation of cementite presents inhomogenous areas inside the microstructure and the coarsening of carbides is more evident on the grain boundaries (Figure 12c,d).

Based on Figure 13, comparing the microstructures of both cryotreatments via scanning electron microscope, in case of shallow cryotreatment, carbides are finer and more homogeneously distributed inside the grains of tempered martensite (Figure 13b), compared to carbides’ distribution inside the grains of tempered martensite after deep cryotreatment (Figure 13a).

4. Conclusions

The microstructure of both shallow and deep cryogenically treated-low tempered AISI H13 tool steel is comprised of a matrix of needle-like lath martensite were globular transition and primary carbides are finely dispersed. Retained austenite was also found in the microstructure. Enhanced density of globular carbides is observed to both sub-zero treatments of the specific tool steel grade [70]. It is triggered by different mechanisms

‧ the relatively prolonged holding time of 24h to -196⁰C in deep cryogenic treatment [20] and the prolonged ramp up time of 30h to -130⁰C in shallow cryogenic treatment respectively.

Tempering on 500⁰C (secondary hardening peak) showed that the microstructure of both subzero treatments is comprized of tempered martensite and carbides. The weight percent of retained austenite is less than 5%. In case of deep cryogenic treatment, the carbide precipitation is more pronounced to the grain boundaries of tempered martensite. On the other hand, in case of shallow cryogenic treatment, carbides are homogenically dispersed inside the tempered martensite grains. Evidently, the parameters of the subzero stage through the heat treatment sequence, namely holding time and ramp up rate are critical for the precipitation topology of the secondary carbides which affects the acquired microstructure and its properties [4].

Main features of the microstructure of both cryotreatments of the tool steel grade and after tempering at 525⁰C are a) formation of cementite within the microstructure and b) carbides precipitation. More specifically, unhomogenous distribution of cementite inside the microstructure is observed in case of shallow cryogenic treated specimen. Through tempering at 525⁰C, in both subzero treatments, distinct carbides precipitation on both inner and boundary areas of the tempered martensite grains is realized.

In general, cryotreatment and tempering at 550⁰C, extensive cementite formation is noted as tempered martensite decomposes further provoking microstructural changes and coarsening of carbides are the main features of the acquired microstructure. After shallow cryogenic treatment, there are areas of inhomogenous formation of cementite. In same time, carbides are coarsened on the grain boundaries while they are finer and more distributed inside the grains. The microstructure after deep cryotreatment is more homogenous for both the microstrucure and the dispersion of the carbides respectively.

Different patterns on the evolution of the microstructure after deep and shallow cryogenic treatment is observed. It is evident that prolonged holding time at deep cryogenic temperature leads to a more homogenous microstructure with equaly developed carbide distribution on both inside and the boundaries of the grains in a matrix of tempered martensite. The prolonged ramp up rate after holding time to shallow cryogenic temperature lead to inhomogenous microstructure having fully developed/coarsened carbides on the grain boundaries while finer, having higher density and finer distribution carbdes inside the grains of tempered martensite.

Thermal hysteresis in case of deep cryogenic treatment and consecutive equilibrium states through warming up in case of shallow cryogenic treatment are the triggering mechanisms of the specific subzero treatments. Based on the evidence, the microstructures acquired are different and will have an impact to the properties of the specific tool steel grade on the macroscopic level.