The word cryogenic originates from two Greek words: “Kryo”, which stands for very cold, and “genics”, which represents to produce. So, cryogenic treatment is a method, where material gets cooled to temperature below 0 °C (<273 K) to induce specific properties of the material [9]. The common names for cryogenic treatment found in the literature are: Cryo, Cryo-treated, subzero, sub-zero, low temperatures treatment, ultra-low temperatures, cryogenics, cryogenic processing, cryoprocessing, cold treatment, cryogenically treated, cryogenating, cryogenic stress relief, cryogenic hardening, and cryogenic thermal cycling treatment [10,11,12,13,14,15,16,17,18]. As various as the naming, also the applicability of cryogenic treatment is numerous, ranging from fabrication processes (3D printed objects [19], electrodes [20], welded joints [21] etc.), down to material removal processes (grinding [22], drilling [23] and milling [24]) and metal forming processes (manufacture of tools or components [25]). In last few years, applications of cryogenic treatment in nanosciences and nanotechnologies of metallic materials are used for automotive applications as a part of the heat transfer applications, where the main aim is to resolve the agglomeration of particles [26]. The applications of cryogenic treatment have evolved throughout the last century, from treating whole components of a machine to individual part of tools or even specific nanomaterial structures for application or integration into other materials. Furthermore, nowadays interdisciplinary research has become more focused on cryogenic treatment to make an advance step in understanding of material manipulation with such treatment in different disciplines or industries, such as medicine, space, music and automotive industry [2,3,4,20,27].

 

Life and performance of a specific automotive component greatly depends on proper design, selection of material, accuracy of production, and application of selected heat treatment, which at the end is subject to production costs [28,29,30]. In automotive industry, ferrous metals (steels) and non-ferrous alloys (aluminum, titanium, magnesium, nickel etc.) are used as components, where both groups can be treated by cryogenic treatment. Automotive components, made of this raw material, are timing gears, crown wheel, crown pinion, bevel wheel, king pin, pinion shaft, bevel pinion, differential turnion, crankshafts, connecting rods, axle, etc. [31,32,33]. The idea of every industry, including the automotive industry, is to reduce costs of manufacturing and to obtain the quality of the final products. For example, in every passenger vehicle there are several parts (some described above) that are continuously exposed to fatigue and wear and are usually hard to produce and need specialized lubrications and proper lubricant to prevent failures [31].

 

Cryogenic treatment can provide solutions to such problems as it improves thermo-chemical, mechanical and tribological properties [17,32], leading to higher obtained product qualities and competitiveness, whilst retaining lower production and maintenance costs. The costs of production are an important factor as it influences the selection of the material and process that determine the possible failure of a material due to different wear mechanisms [33]. DCT can reduce production costs as DCT changes (improves) properties of material or component. Furthermore, a DCT cycle can potentially be used to replace a portion of the tempering cycle(s), which reduces processing costs and minimizes equipment set up (even up to 50% of cost reduction) [15,34,35]. During cryogenic treatment, not only wear reduction, but also other mechanical properties, such as dimensional stability, ductility, hardness, residual stress, toughness, strength and fatigue can be improved [1,7,8,29,36,37,38].

Conventional cryogenic treatment is a gradual cooling of the metallic material or component to predetermined subzero temperature, then maintaining the selected temperature for a specific time and eventually, gradually warming up, to bring material or component back to room temperature at 20–25 °C (293–298 K) [8]. It is commonly recognized that cooling and warming rates of the metallic materials should be kept constant to improve properties and to avoid thermal micro-cracking [28,39]. Cryogenic treatment is an affordable permanent treatment process of metallic material that can be applied only once, but affects the entire cross section of the material and can at the same time replace additional procedure (example: single tempering step instead of double or triple tempering) [34].

 

Cryogenic treatment consists of the following sequence [40,41] (Figure 1):

(1)

Slow cooling to predetermined temperature (descent, slow cooling keeps the stress to a minimum).

(2)

Soaking period for pre-determined time with uniform or cycling temperature profile [41,42].

(3)

Slow warming up to room temperature (ascend; slow warming rate keeps the stress to a minimum).

The cryogenic treatment is divided into three systematically different temperature systems. First and foremost used is conventional cold treatment (CCT), where temperature is reduced down to −80 °C (≥193 K) [1,7]. CCT was common in the past, because it was believed that temperatures till −80 °C (≥193 K) were adequate to transform most of the retained austenite into the martensite in steels, establishing improved fatigue strength and dimensional stability and increased wear performance [40,41]. Shallow cryogenic treatment (SCT) occurs from −80 to −160 °C (193 to 113 K) [34,43,44,45]. In Celsius scale, the deep cryogenic treatment (DCT) is in some literature below −160 °C (113 K) [46] and other literature −153 °C (120 K) [47,48], depending on the corresponding author. DCT can be also found in literature named as sub-zero treatment (SZT) [42] or ultra-low temperature process (ULTP) [18]. In SCT, the metallic material is placed after quenching directly below −80 °C for predetermined period to reach thermal equilibrium. SCT causes transformation of retained austenite into martensite (increase in hardness) and allows size reduction of carbides as well as increases the number of precipitated carbides in the microstructure [49].

DCT of metallic materials is reported to induce the conversion of retained austenite into martensite, carbon redistribution and site reduction [46], reduction of free energy of crystal structure [50], precipitation of fine submicroscopic carbides [22,23,25,29,41,51] and precipitation of secondary and tertiary carbides [46,52,53,54]. DCT is a very valuable tool, when dealing with the improvement of the material’s properties. On the effectiveness of the DCT on selected material and its properties influence different factors such as selected cryogenic temperature (72%), holding time (soaking period) (24%), cooling and warming rate (10%) and sequence of DCT (2%) [8,55,56,57]. The factor importance was based on wear resistance and then calculated statistically with Taguchi method, where the aim was to identify the importance of each critical parameter (cooling/warming rate, soaking temperature, soaking time, tempering time, placement of DCT) and its mechanism [57]. Beside these factors, influence on the performance of DCT have also the material chemical composition [58,59], temperature of austenization or solution treatment [60] and tempering and aging temperature [61]. These individual parameters dictate the efficiency of DCT and the individual evolution of certain phases in treated material, leading to a great impact on the resulting properties of the material. The sequence of cryogenic treatment is usually applied after quenching of metallic material [28,62,63], but there are also studies where the treatment was implemented after annealing [64], or even after tempering [65,66]. In recent literature, a specific type of DCT is described as multistage cryogenic treatment (Figure 1), where treatment consists of repeated rapid cooling down to DCT temperature (below −196 °C) and re-heating up to SCT temperature (−80 °C) or even to room temperatures (20 °C or more) [67,68].

Traditional use of steels in the automotive industry, is a result of formability, longer life, low production costs, adaptability to corrections, susceptibility to modifications in strength level, recyclability, good crash energy absorption and ease of joining [69]. On average, 900–1090 kg of steel (different grades) is used per one single car. This includes body structure (doors, trunk, panels) (34%), drive train (engine block, wear resistance gears) (23%), suspension (12%), the remaining 31% is in tank, tires, wheels, breaking and steering system of the car. The plan for future steel vehicles (FSV) is to reduce body mass of the car to only 188 kg, but at the same to maintain the properties of the previous form body [70,71]. However, usually these new materials for FSV are more expensive, complex (steel, matrix, non-ferrous alloy, composites), have no corrosion resistance, have limited service life and are more difficult to produce, and at the same time it is harder to maintain their properties [71]. Many of these difficulties could be overcome by the application of a variety of new methods dealing with the improvement of steels (coating, DCT etc.).

An overview (Table 1) of the literature indicates the variety of ferrous materials used as reference materials for DCT in research for automotive applications. The materials have been tested and investigated only individually and were conducted without any repeated experiments following up on the presented results of the research.

 

From the summary given in Table 1, the soaking temperatures of applied DCT vary from −150 to −196 °C [3,47,53,72,73,74,75,76,77], depending on each author and research. For example, research from Surberg et al. 2008 [48,77] is already between SCT and DCT, so that interpretation of results cannot clearly provide description of the effect of DCT on the metallic material. The temperature is the most important factor (72%), which influences on the effect of DCT on the selected metallic material and thus it is important to use temperatures that are more consistent to attain comparable results between studies and with it the effect of DCT on a particular material. Holding time (24%) of experiments is in most cases 24 h [72,74,75,76,77], but some divergences can also be found such as 0.5, 2, 16 and more than 40 h [3,53,73]. Holding time is the second factor, which influences on the effect of DCT on metallic material. However, the variety of time periods applied on the steel show that the mechanism and dynamics of DCT is still not well understood, as some studies get the same resulting effect with shorter holding time [3,47,73] and others with longer soaking period. Cooling and warming rate in conducted studies differs from 0.25 [47] to 3 °C/min [53]. Even though the generally accepted recommended cooling and warming rate is 0.5 °C/min [28,39]. This again demonstrates that there is no systematic approach to DCT and thus variety of approaches and explanations are expected and occurring.

Non-ferrous alloys are largely present in a vehicle as components of anti-lock braking system (ABS), body panels, cylinder heads, pistons, power train, indoor structure, transfer case, wheel core, car seat frame, electro-mechanical applications etc. [80]. One of the advantages of non-ferrous alloys is that they are mostly lighter as compared to ferrous alloys. Aluminum alloys are also more corrosion resistant compared to traditional steel components. In recent years, magnesium, nickel and titanium alloys are getting more attention in the automotive industry. Magnesium alloys can be mostly found in racing cars (very light material and contribute to saving fuel), for example, in Porsche 917, 133 kg of its total weight is attributed to components made of magnesium alloys [5]

A synopsis of the literature on the non-ferrous alloys exposed to DCT presented in Table 2 provides interesting outcome, demonstrating that not much research has been conducted in the field of DCT in combination with non-ferrous alloys. This is shown by the scarce amount of studies conducted with a much limited amount of different alloys researched in terms of DCT effect [81,82,83,84,85,86]. The overlook of the results verified, that research of non-ferrous alloys is still, more or less at the beginning, even though some studies on non-ferrous alloys were conducted by National Aeronautics and Space Administration (NASA) in 1950s–2000s of aluminum alloys for space industry [87]. However, due to market demands, the development and need of non-ferrous alloys in automotive, space, medicine etc. industry will reemerge. As a consequence, more studies will be promoted in the field of cryogenic treatment and behavior of selected alloys.

 

Observation showed that selected soaking temperatures of applied DCT on non-ferrous alloys is in the range of −163 to −196 °C, being about 10 °C lower than in average used for ferrous alloys in automotive industry [81,82,83,84,85,86,88]. This indicates, that lower temperatures are apparently needed, when dealing with non-ferrous alloys. The possible explanation could be in the absence of carbon and the much different phase formation mechanisms found in non-ferrous alloys. Soaking time of experiments is in the most cases longer than 20 h [82,85], but there can be found also shorter holding times, for example, 2, 4, 8, 12, 16 h [82,83,88]. This could also indicate that longer time is needed for improvement of the properties of the non-ferrous alloys compared to ferrous alloys. Cooling and warming rates in conducted studies differs from 0.5 [85] to 20 °C/min [82]. The guidelines for application of DCT on non-ferrous alloys are also spare and are not systematically approached.