Assignment 7: Hot Isostatic Process (HIP)

Problem 13.8:

Make a brief literature study of the hot isostatic process (HIP). Discuss the mechanics of the process, its advantages, and its disadvantages. Think broadly about how HIPing can improve more conventional processes, and how it can impact design.

Answer:

The Hot Isostatic Pressing (HIP)  is a manufacturing process used to reduce the porosity of metals and increase the density of many ceramic materials.  HIP is the simultaneous application of high temperature and pressure to metals and other materials for a specified amount of time in order to improve their mechanical properties. In the HIP unit a high temperature furnace is enclosed in a pressure vessel. The temperature, pressure and process time are all precisely controlled to achieve the optimum material properties. Components or parts are heated in an inert gas, generally argon, which applies "isostatic" pressure uniformly in all directions. This causes the material to become "plastic" allowing voids to collapse under the differential pressure. The surfaces of the voids diffusion bond together to effectively eliminate the defects achieving near theoretical density, while improving mechanical properties of the components or parts.

Hot Isostatic Pressing is a well established process for the improvement of a wide variety of materials such as titanium, steel, aluminum and superalloys. Using this process, voids within a casting can be reduced or eliminated and encapsulated powders can be consolidated to create fully dense materials. Also, dissimilar materials can be bonded together to manufacture unique, cost effective components.

 

Advantages:

 

 

• When incorporated as an integral part of the manufacturing process reduces scrap and improves yield.
• Frequently allows replacement of wrought components by castings.
• Reduces quality assurance requirements by improving material properties and reducing property scatter. Often, the savings on radiographic costs will cover the costs of HIP.
• Maximizes material utilization by improving material properties.
• Parameters can be established to minimize subsequent heat treatment requirements.
• Processed parts exhibit higher reliability and longer service life.
• Allows for smaller, lighter-weight high strength parts.
• Can reduce the total production costs of a product.

 

Disadvantages:

 

• Volumetric Shrinkage
• Surface-Connected Porosity – Pressurizing gas will enter pores and hold them open (need to HIP prior to machining).
• Incipient Melting – If a compositional gradient (segregation) exists within a part, the local melting temperature may be lower than the HIP temperature.
• Eutectic Melting – Solid state reactions between parts and support tooling must be considered.
• Creep Deformation – Care must be taken with thin wall section parts.
• Material Cleanliness – HIP can only eliminate internal porosity; other defects such as inclusions will remain.

In HIP, the pressure and temperature are applied simultaneously than conventional process that used pressure unaxially at high temperature. However, the pressure is applied through the isostatic medium and as a result, is transmitted to the material with same load in all directions. HIP can improve more conventional process by improving wear, corrosion and heat resistance.BY using HIP, perfect material design can be obtained with high control of chemical composition, maximum density, isotropic properties and good control of microstructures with small grains.