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Put a Check on Heat Checking with an Enhanced Die Steel

Joe Price, Technical ManagerBohler-Uddeholm Corporation

Elgin, Illinois

Introduction

In recent years, improvements in hot work tool steels, heat treatment, and coatings have positively influenced the longevity and performance of die casting dies. For hot work die materials, these improvements have come from advancements in manufacturing methods which produce cleaner, more homogeneous steels through remelting technologies, as well as improved forging techniques. In addition to manufacturing enhancements, modifications to standard alloy compositions and new alloys composi-tions provide die materials with improved properties. These remelting technologies include Electroslag Remelt-ing, Pressurized Electroslag Remelting, and Vacuum Arc Remelting, with each process providing progressively increased cleanliness. Heat treatment has improved with faster cooling rates, which include back pressure quench capabilities up to 20 BAR. However, with the before men-tioned improvements in hot work die materials and how they are processed to make a die, thermal fatigue cracking (heat checking) still represents the most significant cause for die failure. The issue of heat checking and what can be done to delay its onset is addressed in several published works1, 2 and will be the focus of this article.

In this article, the discussion will cover an enhanced die steel, BOHLER W403 VMR (herein described as Modi-fied 1.2367 VMR), Premium H13, and 1.2367 ESR. A comparison of Modified 1.2367 VMR, Premium H13, and 1.2367 ESR will be made with respect to their properties that resist heat checking. This comparison will include a case study in which the Modified 1.2367 VMR, Premium H13, and 1.2367 ESR were used in the same die casting application. The results of how these specific hot work tool materials resisted heat checking will be presented. In addi-tion, the influence on maintenance over the period of time the dies were in production will be discussed.

Heat Checking

In die casting, thermal cycling of the die occurs with each shot cycle. During a shot cycle, significant thermal shock is created by the thermal cycling. First, rapid heating of the die surface occurs from molten metal being injected into the die. Then rapid cooling is initiated when die lube spray is applied after ejection of the casting. The shot cycle is typically repeated in only seconds. The thermal cycling of a die casting shot creates stress, especially at the die

surface, and over time tempers back the die surfaces. The combination of the stress and tempering back of the die surface leads to thermal fatigue cracking or heat checking. The resulting damage on the surface of the die transfers to the casting as the molten metal penetrates the heat check-ing cracks. Ultimately, heat checking cracks can grow to the point of gross cracking and break out of the steel.

The transfer of the surface conditions to the castings can cause cosmetic issues which require grinding or buffing to make the casting cosmetically and functionally acceptable. In addition, repair or replacement of the die will be needed at some point. The work performed to the castings and the repairs made to the dies represent increased maintenance cost and down time. This is a costly scenario and needs to be minimized for the die caster.

Hot Work Tool Steel for Dies

Key properties needed to resist heat checking in a hot work tool steel for dies are good thermal conductivity, ductility, and temper back resistance. These properties are influ-enced, at least partially, by the chemical composition of the grades. The inherent ductility of a hot work tool steel is also influenced by the production steps.

In Table 1, the thermal conductivity of the Premium H13, Modified 1.2367 VMR, and 1.2367 ESR is listed. Premium H13 has lower thermal conductivity than both 1.2367 grades. As the compositions are similar, the thermal conductivity of both 1.2367 grades is similar at 400°C. The higher thermal conductivity of the 1.2367 grades provides some advantage over the Premium H13 to resist heat checking.

When taking the chemical composition into consider-ation, the key elements to look at are Carbon, Chromium,

Table 1 – Comparison of Thermal Conductivity

Grade Thermal Conductivity(W/m °C) at 400°C

Premium H13 29

Modified 1.2367 VMR 32

1.2367 ESR 31

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Molybdenum, Vanadium and Silicon. Table 2 lists the typical compositions for Premium H13, Modified 1.2367 VMR and 1.2367 ESR. The carbon content of all three hot work tool steels is very similar and will not be a factor for this comparison. Chromium, Molybdenum and Vanadium are strong carbide formers2 and assist in providing temper back resistance by forming more stable secondary carbides during tempering. Lower Chromium and Vanadium in the Modified 1.2367 VMR and 1.2367 ESR grades decreases the amount of primary carbides formed when the steel is produced. The lower Silicon in the Modified 1.2367 VMR also reduces the amount of primary carbides formed. Ductility is improved in tool steels when fewer primary carbides are present.

From the Electroslag Remelting (ESR) process and the Vacuum Arc Remelting (VAR) process, tool steels are made very clean and homogeneous or uniform. In the ESR process, the tool steel is cleaned of unwanted nonmetal-lic inclusions such as sulfides and oxides by reacting with the slag. In addition to cleaning the material further, the remelting process refines the microstructure. Refining of the microstructure is accomplished by the controlled melt-ing and solidification of the ESR process which produces a more homogeneous microstructure in all directions compared to the original cast ingot. The improved micro-structure homogeneity provides uniform properties in all directions and a more robust tool.

The VAR process further improves the cleanliness compared to ESR by being remelted under a vacuum. When remelting under a vacuum, tramp elements and gases such as Phosphorus, Sulfur, Nitrogen, Oxygen and Hydrogen are reduced even further compared to ESR. In Figure 1, the cleanliness is shown by the K0 rating from DIN 50602. The controlled melting and solidification for VAR is also improved compared to ESR which provides an increase in the uniformity of the microstructure and microsegregation. The reduction of the microsegrega-tion is important for the ductility of hot work tool steels. Specifically, reducing microsegregation of Chromium and Molybdenum is documented as improving the perfor-mance of die casting tooling.3 With the further improve-ments from VAR process, there are additional gains in the ductility giving the Modified 1.2367 VMR an edge over the 1.2367 ESR for resisting heat checking.

From the preceding discussion, the Modified 1.2367 VMR should have an advantage over the Premium H13 and

the 1.2367 ESR to resist heat checking. The advantage arises from the modified composition and VAR processing of the Modified 1.2367 VMR, which provides a good combina-tion of ductility and temper back resistance. In addition, the Modified 1.2367 VMR can be heat treated at the same aus-tenitizing temperature as Premium H13, 1885°F (1030°C). No special heat treatment cycle is needed.

Case Study

The following case study involves a die casting application at a large U.S. die caster in which Premium H13, Modified 1.2367 VMR, and 1.2367 ESR were used. Because of the sensitivity related to the specific application, only limited photographs and specifics about the parts being cast are included. Initially, Premium H13 was utilized in the appli-cation, but unsatisfactory performance was experienced with heat checking starting early in the shot counts. The early heat checking led the die caster to seek alternative hot work tool steels to resist the heat checking. The die caster, after consulting with tool steel manufacturers, selected a 1.2367 ESR and a Modified 1.2367 VMR grade to conduct trials.

Process Information

Some of the important die characteristics and process information for the case study are as follows: the size of the casting, the alloy being cast and the process parameters that contribute to increased heat checking. For the casting, the overall weight of the shot was 27.8 lbs, including the runner, gate, and a cast-in steel shaft. The weight of just the casting is 6.9 lbs with a wall thickness of 8 mm across a majority of the part. In the area of the shaft, the thickness is up to 19 mm. The overall mass and wall thickness of this casting cre-ates a situation of significant heat input into the die.

The shot cycle time was 95 seconds and the aluminum temperature as it enters the cavity was 1240°F (671°C). Vacuum assist was utilized to help with filling of the cavity and soundness of the casting. Hot oil was used to maintain internal temperature control of the die at 500°F (260°C).

The die was inserted with a two cavity design. The size of the ejector half was L 25” x W 26.5” x H 7.8” while the cavity half was L 25” x W 26.5” x H 6.0”. Depending on the hot work tool steel, the hardness of the die inserts varied. The Premium H13 and the 1.2367 ESR were 44 to 46 HRC, while the Modified 1.2367 VMR was 46 to 48 HRC. The

Table 2 – Typical Compositions (Wt%)

Grade C Si Mn Cr Mo V

Premium H13* 0.39 1.00 0.35 5.25 1.45 1.00

Modified 1.2367 VMR** 0.38 0.20 0.25 5.00 2.80 0.65

1.2367 ESR** 0.38 0.40 0.50 4.95 3.00 0.55

* median of composition listed for NADCA for Grade B** median of DIN 1.2367 composition

Figure 1 – Graph showing cleanliness improvements with ESR and VAR process as measured by K0 per DIN 50602.

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target number of shots for the design of the die is 80,000 with an estimated annual production of 640,000 pieces.

Results

Die inserts were made from the Premium H13 initially. The inserts started to show extensive heat checking between 35,000 and 38,000 shots. This is well below the minimum 80,000 shots the die was intended to achieve. A photograph of the heat checking transferred to the casting is shown in Figure 2. For the Premium H13, 238 hours of preventive maintenance (PM) was required which included routine stress relieves as 975°F (524°C), assorted repairs and welding. In addition to time spent during the PM, 31 hours in welding was done between PMs. This die insert was retired prematurely at 67,700 shots.

The die inserts made from 1.2367 ESR and Modified 1.2367 VMR were put into service in the same time frame. A review of the castings and dies after approximately 22,000 shots showed the insert made with Modified 1.2367 VMR had exhibited improved protection over the 1.2367 ESR for heat checking. Photographs comparing the same area at two different locations on the castings are shown in Figure 3. There is a noticeable difference in the extent of heat checking.

As the production continued, the PM and cost of weld repair showed the difference in the performance of the two hot work tool steels. A summary of the PM time is listed in Table 3. At 81,000 shots, the insert made with 1.2367 ESR had a total of 919 hours of PM with a total of 1,432 hours of tool room time. For the PM done at 55,500 shots, the insert required extensive weld repair, which required significant time out of production and additional repair cost. In contrast, at 101,000 shots the insert made with Modified 1.2367 VMR had a total of 373 hours of PM with a total of 1,035 hours of tool room time. Over the time for the 101,000 shots, only minor weld repairs were required on the Modified 1.2367 VMR insert. However, at the 101,000 shot count, the insert was retired. Significant weld repair was required for the Modified 1.2367 VMR insert to continue production and it had exceeded the targeted 80,000 shot count.

The die caster had projected that a 17% improvement was needed to justify the increased cost of the Modified 1.2367 VMR over the Premium H13. Compared to the Premium H13, there was a 49% increase in shots. The die caster said they were very pleased with the performance of the Modi-

Figure 2 – Photograph showing heat checking damage on a casting produced in the Premium H13 insert.

Figure 3 – Photograph comparing heat checking damage on a casting produced in A1 and A2 (Modified 1.2367 VMR insert) to B1 and B2 (1.2367 ESR insert).

A1

A2

B1

B2

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fied 1.2367 VMR and stated it far exceeded the needed expectations. Compared to the 1.2367 ESR, the Modified 1.2367 VMR had 16% more shots with 59% less time in PM. The die caster made more inserts with the Modified 1.2367 VMR and was experiencing similar performance at the time the final information was collected for this article.

Summary

Alternative hot work tool steels are available in the market today that will provide improved resistance to heat checking compared to Premium H13. These improvements are partially achieved from higher ductility and better temper back resis-tance found in the alternative hot work tool steels. In addi-tion, as shown by the results of the case study, hot work tool steels with very similar chemistries do not necessarily perform the same. How the tool steels are processed, specifically the remelting process can influence the properties and perfor-mance. This is well known with the improvements realized for H13 when it is remelted. In the case study presented here, the vacuum remelting of the Modified 1.2367 VMR (Bohler 403 VMR) provided better performance over the 1.2367 ESR with lower maintenance cost. The better performance is at least partially attributed to the better ductility of the Modi-fied 1.2367 VMR (Bohler 403 VMR).

References 1 A. Srivastava, V. Joshi, R. Shivpuri, Computer Modeling

and Prediction of Thermal Fatigue Cracking in Die-Cast-ing Tooling; Wear 256 (2004) 38-43.

2 David Schwam, John F. Wallace, Sebastian Birceanu, Effect of Design Factors on Thermal Fatigue Cracking of Die Casting Dies; Work Performed Under Contract DE-FC07-00ID138486, US Department of Energy Assistant Secretary for Energy Efficiency and Renewable Energy, Washington, D.C., October 2004.

3 J. H. and A.M. Schindler, New Materials Study of 5% Chromium Type Steels for Use in Die Casting Dies, 8th SDCE International Die Casting Exposition & Congress in Detroit Michigan, March 1975; Paper No. G-T75-053.

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About the AuthorJoe Price is a Technical Manager at Bohler-Uddeholm Corporation where he provides technical support to customers and the sales team. He is member of the North American Die Casting Associa-tion (NADCA). As part of NADCA, he serves on the Die Materials Committee and the Die Materials Specification/Heat Treatment Task Force. He can be contacted at joseph.price@bucorp.com.

Table 3 – Summary of Maintenance Time Over Production History.

Grade PM Time (HRS)

Total Tool Room Time

(HRS)

Final Shot Count

Premium H13 238 N/A 67,000

Modified 1.2367 VMR 373 1035 101,000

1.2367 ESR 919 1432 81,000

MATERIALIZING VISIONS

1-800-638-2520 www.bucorp.com

ANTI AGEING CONCEPTHeat checking – the premature end for most die casting tools. That doesn’t have to happen.

An alloy optimization process developed by BÖHLER, as well as vacuum melting technology, provides for the high-est degree of purity in BÖHLER hot work steels. What this actually means is considerable improvement in the toughness and heat conductivity. Properties that ex-tend the life cycle of your tools and reduce the cost per part of your tooling.

W403

Component made from BÖHLER W403 VMR

Component made from ESR H11