The effects of cryogenic processing on the wear resistance of grey cast iron brake discs

Abstract

Cryogenic processing has the potential to significantly increase longevity in many automotive components, where friction and wear are major factors in their operation and eventual failure. Cryogenic treatment affects the whole cross-section of the component and is inexpensive compared to other treatment processes. Whilst numerous studies have been conducted on cryogenic treatment of tool steels since the 1970’s, in many cases showing significant improvements in wear resistance, only minimal work has been done in cast irons. In this study the effects of deep cryogenic treatment (93 K) on the wear resistance of grey cast iron (SAE J431 G10) brake rotors was assessed and related to changes in their microstructure using optical microscopy. A comparative analysis using pin-on-disc testing was carried out on brake discs having undergone deep cryogenic treatment and those that had not, using chrome steel balls as the wear inducing body. The results indicate an improvement in the wear rate of grey cast iron of 9.1–81.4% due to deep cryogenic treatment where significant wear has occurred, although there was no significant change in the bulk hardness, matrix hardness or in the microstructure of the material under optical observation.

Introduction

As part of a safety critical system in passenger vehicles, disc brakes and the materials they are made of must meet a number of requirements. They must have high thermal resilience, a stable coefficient of friction with temperature to avoid brake fade, provide a reproducible uniform response as well as wear uniformly during their service life. Clearly, functional reliability is crucial [1].

Disc brakes represent a complex tribosystem, with the brake rotors forced into contact with brake pads that are typically composite materials with a number of additives to enhance their thermal characteristics, held together by a phenolic resin binder [2]. The contact conditions between these components are extreme, with high clamping forces and temperatures in the friction ring capable of reaching 700 °C over long downhill stretches. As around 90% of the kinetic energy of the vehicle is absorbed by the brake rotors as heat, they must be able to rapidly dissipate energy to ambient air [1].

During braking, the temperature rise is significantly determined by the kinetic energy and therefore the mass of the vehicle as well as the heat capacity and therefore the mass of the brake rotor. Intuitively, larger brake rotors result in a lower rise in temperature and less susceptibility to brake fade, where the coefficient of friction between the brake rotors and pads declines significantly with increasing temperature, leading to greater stopping distances [1].

Brake material manufactures therefore have to optimise between the deformation and deflection of brake rotors under braking forces, weight and thermal performance, whilst providing for a long service life (10⁵ km). For these reasons grey cast iron (GCI) has become the material of choice for passenger vehicles, with its castability and machinability being important factors in its widespread use.

Pearlitic grey cast irons are the primary material used in brake rotors for passenger vehicles, usually with small quantities of alloying elements added to enhance desirable properties. The high graphite content of these materials improves thermal conductivity and the large heat storage capacity of GCI allows brake rotors to avoid overheating and prevent brake fade during use. Additives such as chromium and molybdenum give greater resistance to abrasion and improved heat cracking behaviour [1]. The addition of manganese refines the pearlite matrix and improves hardenability whilst manganese and silicon are added as de-oxidants to improve corrosion resistance. Importantly, silicon acts as a graphite stabiliser and prevents the formation of iron carbides. As demonstrated by Hecht et al. [3], graphite flake morphology has a significant effect on the thermal diffusivity of SAE G3000 (now G10) GCI. Using the flash technique they found that for GCI with the same chemical composition, a 50% increase in thermal diffusivity was related to a fourfold increase in the size of graphite flakes. The interlocking nature of graphite flakes in GCI enhances its thermal conductivity [4]. The benefits of using GCIs are a high thermal storage capacity, thermal conductivity and resistance to brake fade coupled with a high degree of castability and machinability, lending to low manufacturing costs. The main drawbacks of these materials are the large and therefore heavy brake rotors needed, as well as poor corrosion resistance.

In recent years for passenger vehicles and previously for niche and high-performance applications, other types of materials have been studied and used as replacements for GCI. Cueva et al. [5] investigated the possibility of using compacted graphite iron (CGI) as a replacement for GCI. During comparative pin-on-disc testing it was shown that the same friction force, temperature and similar wear rates were found in CGI at lower contact pressures than were required for GCI. Other alternatives include metal matrix composites (MMCs), carbon–carbon (C/C) composites, ceramic matrix composites (CMCs) as well as titanium (Ti) alloys.

The most popular MMCs suggested for replacing GCI in brake rotors are those based on aluminium, as Al-MMCs demonstrate similar tribological performance whilst offering significant weight savings. Unfortunately it suffers from a low melting temperature, limiting its applications [6]. C/C composites are commonly used in motor racing and aircraft applications, where resistance to extreme temperatures and the lowest possible weight are demanded. Apart from the inherent cost of these materials, C/C composites show poor braking performance at low temperatures [7]. CMCs are a more viable alternative, demonstrating significant weight savings over GCI as well as thermal stability up to 1300 °C. These are notably used in performance and luxury vehicles, as well as for aircraft brake systems, but are still prohibitively expensive for widespread use in passenger vehicles [8].

CMCs such as carbon fibre reinforced ceramics with a matrix containing silicon carbide (C-SiC), possess the following benefits when compared to GCI: improved resistance to abrasion and service lives of up to 300,000 km, a two-thirds reduction in weight, high thermal stability and corrosion resistance. However, it is also prohibited from more widespread use due to the complex and costly manufacturing processes required to produce brake rotors. These drawbacks are common with all potential lightweight alternatives to GCI, meaning it is likely to remain the primary brake rotor material in passenger vehicles in the near future [1]. With all composites, there are also environmental and health concerns regarding the production of fine particles during braking that have yet to be sufficiently addressed [9].

Ti-alloys have also been investigated as a lightweight replacement for GCI brake rotors. Blau et al. [10] obtained friction coefficient and temperature data from two commercial Ti-alloys, four Ti-based hard particle composites and a thermally spray-coated Ti-alloy dragged against several commercially produced lining materials. They determined that the wear rates of the Ti-MMCs were greater than that of the reference cast iron used, but lower than those of the commercial Ti-alloys. The thermally spray-coated Ti-alloy exhibited least wear and was recommended for further investigation.

Casting grades of grey iron specified by SAE J431 are commonly used in braking applications. The nominal composition of SAE J431 G10 (used in this study) is given in Table 1.

Cryogenic processing or cryogenic treatment, often simply referred to as ‘cryotreatment’, affects the entire cross-section of materials and components and is typically applied as a single additional stage to the conventional heat treatment cycle. Cryotreatment involves lowering the material to sub-zero temperatures and holding or ‘soaking’ for a defined period of time before raising the material back to ambient temperature. The objective of cryotreatment is therefore to cause permanent changes to the microstructure of a material that enhance desired properties, with minimal or insignificant adverse effects.

The parameters that are typically controlled and varied during cryotreatment are: cooling rate, soaking temperature, soaking time, heating rate and the position of the cryotreatment within the overall treatment cycle in cases where materials or components are to be tempered. The statistical significance of parameters in optimising wear resistance through deep cryogenic treatment of a high-chromium martensitic steel was investigated by Darwin et al. [11] by applying Taguchi Design of Experiment methods. They determined that the significance of parameters was as follows: (1) soaking temperature, 72%; (2) soaking time, 24%; (3) rate of cooling, 10%; (4) tempering temperature, 2%; and that the tempering period was statistically insignificant.

Cryotreatment typically involves the use of nitrogen that is fed into a thermally insulated tank containing the parts or materials to be treated. The nitrogen can be used in one of three ways: as a liquid that parts or materials are immersed in to perform a ‘cryo-quench’; as a working liquid for cooling air within the tank; or as a gas that is blown through the tank by fans, creating a nitrogen atmosphere. Cryo-quenching may cause thermal shock in parts or materials with large volumes as the outer material cools much more rapidly than that within [12], whilst the use of liquid nitrogen as a coolant for air can lead to the build up of vapour ice on the surfaces of components in less than ideal conditions. Using gaseous nitrogen to create a nitrogen atmosphere within the tank has neither of these drawbacks [13] and allows for controlled cooling of parts and materials.

Sub-zero treatments are classified by their soaking temperature as either cold treatment (CT: 273–193 K), shallow cryogenic treatment (SCT: 193–113 K) or deep cryogenic treatment (DCT: 113–77 K). These ranges have been established due to their association with the cooling agent used; dry ice for CT and liquid nitrogen for SCT and DCT [14].

One of the key commercial factors supporting the adoption of cryotreatment is its relatively low cost, both in terms of the equipment required and running costs per treatment cycle. Whilst a number of companies treat small batches of finished components for automotive, aerospace and manufacturing industries, as well as treating more novel items such as sports equipment, cryotreatment is not yet an industrial process despite reported evidence indicating substantial improvements in wear resistance, hardness and fatigue scatter reduction. The reasons for this lie in the lack of any cohesive understanding of the physical effects of cryotreatment on the microstructure and composition of treated materials, although a number of substantial studies have been carried out [15].

Investigations into the effects of cryogenic treatment have been largely confined to tool steels, which began in earnest with the wide ranging study of Barron [16]. Twelve tool steels, three stainless steels and four others were subjected to both DCT (77 K) and SCT (189 K) and their abrasive wear resistance determined using a block-on-ring experimental setup. It was found that the wear resistance of these alloys increased by up to 718% (in the case of AISI D2 steel) due to DCT, whereas in others (including AISI A2, A6 and T2 steels) no significant changes were observed.

Subsequent investigations have reported an extremely wide range of improvements in wear resistance of tool and alloy steels due to DCT. In tool steels these are typically associated with the following phenomena: (i) the reduction or elimination of retained austenite (γ_R) from the microstructure [17], [18]; (ii) increased precipitation dispersion and refinement of secondary carbides [19]; and (iii) the increase in matrix hardness due to DCT, although increases in bulk hardness are also reported for some tool steels [20], [21]. The significance of each of these factors is difficult to quantify due to the range of material compositions under investigation. However, comprehensive studies such as those of Das et al. [21], [22] are beginning to define the link between the effects of cryogenic treatment on wear resistance with microstructural changes in specific materials.

Laboratory investigations into the effects of cryogenic treatment on the effects of AISI M2 high-speed steel are backed up by machining tests carried out by Firouzdor et al. [23] who found that tool life of AISI M2 HSS drills were improved by 77–126% due to DCT when drilling blind holes in carbon steel blocks.

Significant differences exist between the composition of GCI and tool steels. Its largely pearlitic matrix (>95%) effectively precludes the possibility of the enhancement of its wear resistance by cryogenic treatment due to the transformation of retained austenite to martensite. Of the other mechanisms suggested for tool steels, it remains to be seen what reaction graphite flakes will have to DCT temperatures, though it is suggested that increased carbide precipitation and refinement remains a possibility due to the large carbon content in cast irons.

This preliminary investigation is to determine what effects cryogenic treatment does have on the wear resistance of SAE J431 G10 GCI brake rotor material and relate these to the changes observed in its microstructure. This simulation is a comparative study looking at the effects of cryogenic treatment on a specific material, and does not attempt to replicate realistic braking conditions in terms of the material pair, loading, contact geometry or sliding speeds.