Upgrading castings by hot isostatic processing.

Upgrading Castings by Hot Isostatic Processing

Hot Isostatic Processing (HIP) was invented almost 35 years ago. Since then it has grown from a laboratory curiosity to an important commercial process that is used in many areas of metallurgical processing. HP also has important ceramic processing applications.

HIP is a process in which components are subjected to the simultaneous application of heat and high pressure in an inert gas medium. The pressure is uniform in all directions, or isostatic.

HIP was initially conceived for the purpose of cladding nuclear fuel elements. Support for the development work was funded by the U.S. Atomic Energy Commission. In its early stages of development, it was expected to be limited to relatively low volume uses; however, it is now possible to process several tons of material in a single cycle that generally requires only a few hours.

Several large HIP facilities are operating around the world today, with current applications in the following five areas.

Upgrading of Castings--HIP is used rather widely in the production of high integrity castings. The process closes internal porosity, thereby improving the properties and reliability of castings.

Consolidation of Powders--Powders are consolidated by HIP to various intermediate and finished products, including billets to be further worked by forging and rolling, from which parts are machined.

High Quality Structural Ceramic Parts--HIP is used for the production of near net shape parts of structural ceramics, such as silicon nitride, silicon carbides, aluminides, etc.

Rejuvenation of Fatigue Damaged Parts--It has been shown that HIP can be used to extend the fatigue life in fatigue damaged aircraft components.

Diffusion Bonding--Originally, the process was developed to produce larger components of small Zircalloy clad pin type nuclear fuel elements.

Of these five applications, the greatest current commercial application is the upgrading of castings. This article reviews the background of HIP, indicates its advantages in foundry applications, reviews the current state of equipment and cites the economic advantages of HIP.

History of HIP

HIP was developed 35 years ago to fabricate complex fuel elements in nuclear reactors. About the same time, HIP began to be used to consolidate metal and oxide powders into complex shapes, also for nuclear applications.

However, as with most newly developed processes, costs were high. As a result, most work using the process involved only very expensive and difficult to fabricate materials and generally was limited to a few relatively low volume applications. The small volume capabilities further limited the uses of the process. Available equipment also was expensive to use.

During the mid-1960s, several developments allowed HIP to be used in broader applications. Perhaps most significant was the development of advanced designs for HIP furnaces. These furnaces allowed processing of large components by HIP for the first time. For example, furnaces with hot zones of five ft or more in length were developed, permitting HIP of parts as large as full sized nuclear fuel elements.

Later in the 1960s, HIP was given another significant boost as a production tool by the almost simultaneous development of two processes--gas atomization of powders and rapid turnaround HIP equipment. Gas atomization allowed high volume production of relatively low cost, high quality, pre-alloyed powders of many alloys, permitting small and large parts to be made. With the development of rapid turnaround equipment, HIP became economically competitive with more conventional powder processing techniques.

In the early 1970s, several HIP facilities were constructed in the U.S. and in several other countries as well. These had a wide range of sizes and were designed for both research and production applications.

In 1972, Battelle and Autoclave Engineers, Inc, undertook another very significant development of HIP as a commercial tool. This was the design, construction, installation and operation of the world's largest HIP system. Battelle operated the system for more than two hundred cycles and then transferred it to Crucible Steel Co. Currently, they are operating it successfully for commercial applications.

The work zone of this large system measures approximately 48 in. in diameter by 108 in. long. This facility was particularly significant because it demonstrated for the first time that it was possible to process tonnage size loads in relatively short periods of time and for low costs.

Since the mid-1970s, several additional applications of the HIP process have been found and demonstrated. In 1974, it was first demonstrated that HIP could be used to eliminate internal defects in metal castings. This is the application of most immediate value to the foundry industry.

HIP System

Basically, HIP involves the simultaneous application of pressure and temperature to a part over a period of time. In essence, the part is squeezed isostatically at an elevated temperature.

Generally, the pressure is applied by gas, usually argon. Pressures up to approximately 30,000 psi are used. Temperatures are 930-2350F (500-1300C) for metals and up to 3630F (2000C) for ceramics.

The major element of the processing system is a cold wall pressure vessel, installed below floor level. Figure 1 shows how the HIP furnace, usually a resistance heating type, is mounted internally in the pressure vessel and is thermally isolated from it. The entire HIP system includes gas storage and pressurization systems, power supply, data acquisition system, process control equipment and, where appropriate, external material handling systems.

Casting Upgrading

Cast alloys are subject to defects that generally result in their having lower and more variable mechanical properties than their wrought counterparts. These defects include shrinkage and gas porosity, hot tears, inclusions and alloy segregation.

Reasonable control of these defects is possible by proper mold design and good foundry practice. However, the complete elimination of other defects, notably shrinkage, from cast shapes is not always possible without applying some external force to accomplish the deformation necessary to close voids and porosity.

HIP provides the ideal mechanism for the application of this driving force. The simultaneous application of heat and pressure combine to collapse voids and porosity by creep mechanisms and plastic deformation and to "heal" the material by diffusion bonding the void surfaces together.

The isostatic nature of the applied pressure is well suited to healing defects in castings. Void closure is accomplished with minimum, often not measurable, distortion. Workpieces of complex geometry can be treated without the use of complex or expensive tooling.

The many investigations conducted on several classes of materials show that HIP results sometimes in startling improvements in mechanical properties, as well as significantly reduced scrap losses and decreased rework and weld repair requirements.

Of high significance is the marked reduction in the statistical spread or scatter usually associated with cast material properties. Minimum observed values are usually increased. The net result is improved reliability and efficiency of materials utilization in cast components. HIP also can render castings fit for applications formerly requiring more expensive forged or wrought and machined materials to meet design specifications.

In one recent study, Dr. B. A. Rcikinson, HIP (Powder Metals) Limited, U.K., conducted a series of trials with cast and heat treated low alloy steels to establish optimum HIP parameters and properties. Two HIP parameters were tested on air-melt cast test bars of En 40B (BS 722 M24). The composition of this cast alloy is: C, 0.29; Si, 0.32; Mn, 0.61; S, 0.035; P, 0.043; Ni, 0.13; Cr, 3.1; Mo, 0.55; Cu, 0.29.

Properties of as-cast/heat treated test bars are compared with properties obtained with HIP/heat treated test bars in Table 1. Results show the improvements in tensile strength, elongation and impact properties.

A further series of trials involving En 40B established the potential to approach wrought strength levels using the optimized HIP treatment. These results are shown in Table 2. Despite the inclusion of a normalize/anneal treatment to homogenize the as-cast test pieces, HIPing still improved elongation properties by about 30%.

By incorporating HIP as a part of the manufacturing process, casting producers and users are beginning to reap many benefits. Production yields are increased as the quality of HIP castings grows. Perhaps most importantly, casting manufacturers who use HIP are experiencing greater freedom in producing their products. Mold design can be simplified to save material formerly used in complex gating and the placement of chills becomes less critical.

Also, alloys once considered uncastable because of problems with hot tears or the formation of undesirable phases during solidification now can be redissolved by HIP. In other words, a concept of cast-to-fill and HIP-to-densify can be developed to take full advantage of hot isostatic processing.

Future Applications

Since its development 35 years ago, HIP has only partly matured to a stage where it is used on an industrial scale. For further development, education as to what the process has to offer is essential. While the extent of HIP applications cannot yet be defined, the demonstrated benefits of HIP make it important to identify areas where technical or cost advantages might be realized.

Early problems with HIP equipment have been solved. HIP equipment is now considered to be a production tool capable of high utilization and quick turnaround times.

The use of HIP for castings will certainly increase. Fine grain superalloy castings, composite materials and directionally solidified castings required to meet increased operational conditions will be "HIPed" more often. Also, diffusion bonding by HIP will come into use for ceramic to metal bonding.

HIP is just now beginning to reach a growth spurt that will propel use of the process through the 1990s, as manufacturers realize the many areas in which HIP can help.