Alloy steel and white iron castings offer unparalleled strength, wear resistance, and versatility in a wide array of applications in the mining industry. These materials have consistently demonstrated their utility in the face of extreme operational demands. Yet, like all engineering materials, alloy steel and white iron castings are susceptible to failure and the consequences of such failures can be significant, ranging from costly downtime to safety hazards.

The definition of failure is the inability of a manufactured part or assembly to perform its intended function for any reason during its service life. Metallurgical failure analysis is a key discipline in materials science and engineering, dedicated to unraveling complex factors that contribute to the unexpected deterioration of components. In the case of alloy steel and white iron castings, the importance of robust failure analysis cannot be overstated. Understanding the origins and mechanisms of failure in these materials is indispensable for preventing future problems, optimizing casting processes, and ensuring the reliability of critical mining operations. Superficial evaluation of “broken casting pieces” is not sufficient in most cases to properly identify a cause of failure.

This paper describes the failure analysis process in the context of alloy steel and white iron castings. It will describe forensic methodologies employed in evaluating casting failures from initial inspection of fractured castings to identification of possible root causes, with a focus on the role of microstructural examination and material properties. Case studies are presented that explain complexities inherent with failure analysis that lead to valuable insights based on an understanding of metallurgy, material behavior, and casting processes.

Application And Alloys

Mineral ore processing is a fundamental stage in the extraction of valuable minerals and metals from their naturally occurring ores. This crucial step in the mining industry involves a series of physical and chemical processes designed to liberate the desired minerals from the surrounding rock and impurities. The main objective is to maximize the recovery of economically valuable elements while minimizing waste generation and environmental impact. A generic example of a comminution process is shown in Figure 1.

Mineral ore processing encompasses several stages to achieve a specific particle size needed for concentration, where techniques such as flotation, gravity separation, or magnetic separation are employed to separate valuable minerals from gangue materials.
Primary and secondary gyratory crushing are the first comminution stages, followed by processing through rotational AG, SAG and ball mills. Primary/secondary crushing is typically performed using austenitic manganese alloys, which are not discussed in this paper.

Grinding ball media are mixed with mineral ores in SAG and ball mills. No grinding media are used in AG mills. Water slurry containing mineral ore enters the feed end, is ground within a circumferential row of shell liners, and exits at the discharge end. Each liner has a wear side impacted by grinding media and mineral ore and a fit side that mates to the mill shell wall. A central lift feature scoops ore and grinding media to a desired height which then falls onto flat plate areas of the liners where ore is crushed. Liners will be gradually consumed by abrasive wear and need to be replaced after a designed service life. Liners that experience cracks, fractures and high wear rates in service are regarded as failures.

Alloy selection is critical to maximize service life during all stages of mineral ore processing. Cr-Mo alloy steel types used at ME Global for AG and SAG mills are described in ASTM A781. White iron alloys for ball mills are described in ASTM A532 Class II – Type B. Pearlitic Cr-Mo alloy steel microstructure is shown in Figure 2 and high chromium white iron microstructure is shown in Figure 3. These micrographs show typical wear surface cross sections from worn liners that successfully exceeded their designed service life.

Failure Analysis Process

When conducting a failure analysis, attention to detail is paramount. A detailed forensic approach ensures that, even years down the line, reviewers of the report will have a comprehensive understanding of the methodology and actions undertaken during the analysis. Timely responses are a customer expectation, and proficiency in the failure analysis process improves analysis skills with each case. A written failure analysis report should be provided for review in a timely manner. However, thorough analysis and accurate results should be the primary focus.

In-Field. Visual examination is the most important aspect of any failure analysis investigation and is a required first step at the in-field site. Many customer mines are remote, and it is important to collect relevant onsite background information up-front. This information is critical in determining causes of failure. Important information may include but is not limited to:

• Photos of parts & failure location within mill.
• Standard/Non-standard operating conditions.
• Overall mill condition/appearance.
• Fit-up assembly process.
• Bolt tightening process and torque levels.
• Run time of part/service life.
• Throughput of material & ball charge level.
• Material being processed.

In many cases, the individual conducting the failure analysis is not the same person that observes the failure at the customer site. This makes gathering critical information difficult at times. Good quality in-field photographs are important to tell a comprehensive story. Customer removal of failed castings and proper material shipping/handling to a qualified laboratory is crucial to avoid post fracture damage of broken pieces. Failure analysis is similar to forensic analysis of a crime scene. Contamination of fracture faces and poor documentation of part condition will hinder the failure analysis process and could lead to incorrect root cause determination.

Laboratory. Once the failed casting(s) or fractured piece(s) arrives at a qualified laboratory for analysis, it is important to take “as-received” photos of all parts and conduct a detailed visual examination. Documentation of every relevant observation makes writing the final report much easier. Casting traceability and proper documentation of broken pieces can be a difficult task with large castings, but this information is critical for investigation of processing records. As always, safety must be a top priority. Measurement rulers and/or tape measures need to be included in all photos to document sizes and locations of all features in the image.

Red oxide rust and debris may be coated on fracture surfaces due to prolonged post-fracture exposure to the environment. Wherever possible, these coatings must be removed to expose fracture surface features. After fractures are clean, the next important step is to take photos of the castings once again. The images of the overall casting may reveal obvious features that help tell the story of what happened. Examples of this include excessive abrasion loss, gouging, and grinding ball “peening” impact damage. Photos of casting identification serial numbers, drawing numbers, and manufacture dates provide production history traceability.

Fracture features can be best exposed and photographed using oblique diffuse lighting. This illumination may reveal fracture initiation sites. In some cases, fracture faces may be severely worn post-fracture and initiation sites cannot be revealed. Knowledge of how the liner was used in service may help determine critical areas for analysis when fracture features cannot be revealed.

The next step is destructive sectioning and material property testing of the casting at critical locations. These locations may include fracture initiation sites but also areas away from the fracture. Due to the large size of these castings and timeliness associated with the failure investigation process it is important to develop a sectioning plan that can deliver the highest level of information in the shortest possible time. Only critical locations must be investigated. All sample sections must be large enough for chemical analysis, hardness measurement and microstructural examination.  Other samples may be needed for impact testing or wear rate measurement. 

At ME Global–Tempe, chemistry is determined by optical emission spectroscopy (OES) according to ASTM E415. Laboratory Brinell (HBW) hardness profiling near and away from the fracture is performed according to ASTM E10 using a 3000 kg applied load and a 10mm diameter tungsten carbide indenter.

Standard metallographic sample preparation according to ASTM E3 is critical for examination of alloy steel and white iron microstructures. Samples do not need to be mounted but mounting is recommended for preservation of edge features. Cross sections can either be hot or cold mounted in appropriately sized molds. Standard procedures for metallographic grinding and polishing should be used to ensure consistent results every time.

Samples should be manually examined by optical microscopy at low magnification in an as-polished condition prior to etching. This is done to reveal microporosity and inclusion features that may be obscured by etching. A 2% Nital etch works well to reveal microstructural features in alloy steel and white iron castings. Digital images acquired over a 12x – 1000x magnification range have sufficient resolution to reveal fine features and identify metallurgical phases present in alloy steels and white iron. All micrographs must contain a scale bar to show the approximate size of features contained in the image. Small amounts of microporosity and inclusions are typically found in all castings and are not necessarily the root cause of a failure.

Two other analytical techniques that are often used in failure analysis are scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). High resolution SEM digital images can reveal fine features on fracture surfaces. Atomic number contrast Backscatter Electron (BSE) images are typically used to reveal microstructural phases and inclusions. X-ray EDS microanalysis is commonly used in failure analysis for qualitative elemental analysis across an acquired image field, but it can also be used to obtain semi-quantitative results at localized areas. Combining SEM and EDS microanalysis methods can be used to characterize specific locations at fracture initiation sites.

Report. Fact-based report writing in failure analysis holds great importance as it serves several critical functions. It documents the entire investigative process, providing a clear and comprehensive account of the methods, findings, and conclusions. This documentation is essential for transparency and accountability, ensuring that all stakeholders have access to the same information. 

CASE STUDY 1: Fractured White Iron Liners From Ag Mill

A 32’ diameter x 13’ long AG Mill at a copper mine had newly installed white iron head liners that fractured after several hours in service. Nineteen fractured liners were identified. Due to customer time constraints, five liners were selected for failure analysis. The nineteen fractured liners included five different part designs and one sample from each design was selected. All nineteen fracture faces were examined & photographed prior to sample selection.

Findings. Bolt hole shoulders showed evidence of wear damage indicating possible alignment issues during installation. Excessive bending stresses imposed on the fit side may have been caused by mill shell mating surface conditions, mill shell fit-up and bolting procedures during installation.

Fracture morphology indicated all fractures initiated from the fit surface of each casting. Fractures were brittle in nature, which is typical for white iron, and initiated either near a bolt hole or at locations of high stress as shown in Figures 4 and 5. (page 33)
At some locations, each analyzed sample had chemistry, hardness, and microstructure within specification for heat treated white iron castings. No evidence of entrapped debris, abnormal microporosity, or large inclusions were found near the initiation sites that would have facilitated crack initiation.

Recommendations. Previous investigations at this copper mine have shown head liner castings have historically fractured in a similar fashion. All breakages occurred soon after installation and initiated from the fit side. To avoid the risk of future failures, recommendations were to review all aspects of the installation process including visual examination and laser scanning of mill shell mating surfaces, laser scanning mill liner fit surfaces prior to installation, and reviewing bolting procedures. Note that similar breakages of head liners were previously occurring in another AG mill at the same copper mine, and a re-designed head liner was produced and installed, which eliminated breakage. The same redesigned head liner could be installed in this mill.

CASE STUDY 2: Worn Alloy Steel Sag Mill Middle Liners

A 28’ diameter x 15’ long SAG mill at a precious metal mine had several rows of alloy steel middle shell liners that experienced significant wear on one side of the liner during the final month of service life. No fractures had occurred. One liner was selected for failure analysis. Mine site admitted the mill was operated under abnormal service conditions near the end of service life.

Findings. Abnormal wear was likely caused by the milling environment and was not attributed to casting quality.

Middle shell liner production records for the liner serial number submitted for analysis did not reveal non-conformances that would have contributed to accelerated and uneven wear in service. Chemistry, surface hardness and nondestructive testing (NDT) inspection results reviewed by the foundry all showed conformance to alloy specification and inspection requirements. Uneven wear shown in Figure 6 did not occur on feed end or discharge end alloy steel liners that also met quality specifications. No abnormalities were noted by visual inspection.

As shown in Figure 7, microstructures of the thickest and thinnest sections of the casting consisted of fine pearlite consistent with properly heat treated alloy steel castings. Through-hardness and chemistry of these sections were typical for this alloy.

Recommendations. No obvious cause for the accelerated wear could be determined. Abnormal mill operation by the customer should be avoided. Design modifications to increase lifter height could be considered to extend mid shell liner service life.

Conclusion

Visual examination is the most important aspect of any failure analysis investigation and is a required first step. Through proper use of failure analysis and forensic methods it is possible to draw correct conclusions and make recommendations based on relevant background information and fact-based evidence obtained from material testing and metallographic examination. Logical progression from a “macro” understanding of background information and visual examination to a “micro” understanding of localized microstructure and material properties is required for all investigations.

Large part sizes and complex patterns associated with alloy steel and white iron liner castings used in mineral ore processing can be handled safely and appropriately to ensure the traceability of all sectioning and destructive material testing.  Proper documentation throughout the entire analysis process is necessary to prevent additional wasted time and effort, and facilitate report writing. Detailed reporting provides a written record of the failure event and is required for further review to identify root causes and make corrective actions to minimize the risk of future failures.