MACHINABILITY OF HEAT-RESISTANCE SUPERALLOYS (HRSA)

MACHINABILITY OF HEAT-RESISTANCE SUPERALLOYS (HRSA)

Heat resistant super alloys is the most challenging material group in machining, have a high temperature, strength, creep, and corrosion resistant material find its application in the industrial turbine, atomic, submarine aircraft sectors. Also, have the most expensive metals to purchase. In addition, the important properties are needed generally utilized in heat exchangers, atomic reactors, and turbine blades.

Although not as popular in machine shops as steel or aluminum, due to the high material and manufacturing cost, it’s an important niche to master. The machinability ranges from 5% to 40%.

Heat-resistance superalloys are a group of materials engineered to have very high strength and superb corrosion resistance. These alloys must also preserve these properties at very high temperatures and chemically hostile environments. They are mainly used in:

Jet engines

Turbines

Oil & Gas equipment

Medical implants

Introduction

Leading manufactures of heat resistance superalloys (HRSA):

Special Metals Corporation:Inconel / Incoloy/ Nimonic / Monel / Udimet

Haynes International:Hasteloy / Haynes / Ultimet

Kennametal:Stellite

Raytheon Technologies:Waspaloy

One of the main factors affecting machinability and alloy specifications is Hardness. The range is extensive, but many popular superalloys come in the range of 32-45 HRC.

Another critical factor is the low thermal conductivity that causes most of the heat to “stay” in the cutting zone instead of being absorbed by the chips and the workpiece.

Machinability Of Nickel (Ni) Based Superalloys

Nickel-base super alloys (Inconel) are generally known to be one of the most difficult materials to machine because of their high hardness, high strength at high temperature, affinity to react with the tool materials, and low thermal diffusivity and they are the most widely used alloys in this group.

• The main feature is excellent strength across a wide temperature range combined with good corrosion resistance.
• HRSA’s are used mainly for aircraft jet-engine parts and in the oil & gas industry.
• The machinability ranges between 9% to 45% depending mostly on the hardness of the material.
• The hardness ranges between 5 to 44 HRC, but most used materials have a hardness of 32-42 HRC.
• The most popular material in this sub-group is Inconel 718, which has a machinability of 10%.

Machinability Of Cobalt (Co) Based Superalloys

Their design is aimed primarily at improving elevated temperature strength by use of solid-solution- and carbide-strengthening mechanisms. Such mechanisms must tolerate substantial additions of chromium (above 20 wt.%) in order to obtain a satisfactory oxidation resistance and a good hot corrosion resistance. Cobalt-based superalloys excel in their wear resistance and chemical stability in harsh and hot conditions.

The design of cobalt superalloys, which is aimed at an enhancement of both their oxidation resistance and their hot corrosion resistance, has received considerable impetus recently, especially since the advent of overlay coating techniques and the extensive studies which have been undertaken to elucidate the hot corrosion mechanisms and the effect of alloying elements.

The most popular materials in this sub-group are Stellite 6 & 21 with a 32-36 HRC hardness and machinability of around 18%. It is the most difficult to machine superalloy sub-group, generating very high wear on the cutting edges. The machinability ranges from 5% up to 20%.

The cobalt-based alloys have been in use for several decades in the manufacturing of various components, they are mainly used in valves and fittings in an acidic environment and medical implants such as artificial hip joints.

Machinability Of Iron (Fe) Based Superalloys

Iron-based superalloys are characterized by high temperature as well as room-temperature strength and resistance to creep, oxidation, corrosion, and wear. Wear resistance increases with carbon content. Iron-based superalloys are a more economical alternative to nickel-based alloys. They provide the same advantages but to a lesser extent and at a lower price.

The AISI 600 series of superalloys consists of six subclasses of iron-based alloys:

• 601 through 604:Martensitic low-alloy steels.
• 610 through 613:Martensitic secondary hardening steels.
• 614 through 619:Martensitic chromium steels.
• 630 through 635:Semi austenitic and martensitic precipitation-hardening stainless steels.
• 650 through 653:Austenitic steels strengthened by hot/cold work.
• 660 through 665:Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.

Maximum wear resistance is obtained in alloys 611, 612, and 613, which are used in high-temperature aircraft bearings and machinery parts subjected to sliding contact. Oxidation resistance increases with chromium content. The martensitic chromium steels, particularly alloy 616, are used for steam-turbine blades. They are used mostly on less critical components that still require heat resistance properties. This subgroup’s most popular material is A-286, with a hardness of 25 HRC and 25% machinability rating.

Boosting machinability of superalloys with SiALON ceramic inserts:

SiaLON is a silicon-nitride-based ceramic cutting material combined with aluminum and oxides. SiAlON cutting tools have some special features such as good fracture toughness, hardness (even at high temperatures) and resistance to sudden temperature changes, which make it a suitable material for machining of various difficult to cut materials including nickel alloys, it’s also making it an ideal option to machine HSRA alloys at much higher cutting speeds than conventional carbide inserts. For example, Inconel 718 that can be turned with a good carbide grade at a cutting speed of 140 SFM (45 mm/min), could be machined with a SiALON turning insert at a cutting speed of 700-800 SFM (240-250 mm/min) There are several kinds of SiAlON ceramics as an example α-SiAlON, β-SiAlON and their combination (α+β). The most commonly used compositions at present are β-SiAlON and (α+β) SiAlONs, which contain a substantial excess of sintering aids. However, the field is still changing with compositions developing to suit specific applications.

Boosting machinability of superalloys with high-pressure coolant

Tool life generally increased with increasing coolant supply pressure. This can be attributed to the ability of the high-pressure coolant to lift the chip and gain access closer to the cutting interface. This action leads to a reduction of the seizure region, thus, lowering the friction coefficient which in turn results in reduction in cutting temperature and component forces. Chip breakability during machining is dependent on the depth of cut, feed rate and cutting speed employed as well as on the coolant pressure employed. To achieve the maximum effect, proper coolant implementation is crucial, and you have to follow two steps:

• Use a tool with internal coolant delivery and an outlet as near as possible to the cutting edge, pointing directly at it. All the leading tool suppliers have dedicated tooling lines for this purpose.
• Add a high-pressure coolant pump to your machine with at least 70 bar (1000 PSI). Inconel 718 that can be turned with a regular tool and pump at a cutting speed of 140 SFM (45 mm/min), could be machined with a proper tool and pump at a cutting speed of 280 SFM (90 mm/min).