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

    Heat-resistant superalloys (HRSA) include a number of high-alloyed iron, nickel, and cobalt-based materials and it’s defined according to the primary alloying element. They all share excellent heat and corrosion resistance, but each sub-group is better or worse in specific properties and used accordingly in different applications.

    Many of the superalloys are Proprietary names owned by a handful of steel manufactures that develop these materials. Besides the chemical composition, the manufacturers guarantee all the material’s mechanical and physical properties in a wide range of temperatures

    They are very similar to the ISO M materials but are much more difficult to machine. With such a wide spread of materials under the generic heading of HRSA, the machining behavior can vary greatly even within the same alloy group.

    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%.
    Material Hardness Machinability SAE DIN
    Hastelloy C-276 86 HRB 20% G-NiMo30
    Hastelloy X 88 HRB 18% 5536 NiCr22FeMo
    Haynes 556 26 HRC 19% 5768 X12CrCoNi2120
    Haynes 625 29 HRC 17% ASME SB443 NiCr22Mo9Nb
    Haynes X-750 36 HRC 13% 5542
    Incoloy 903 41 HRC 11% NiFe42K15Nb
    Incoloy 925 32 HRC 15%
    Inconel 050 36 HRC 13%
    Inconel 625 29 HRC 17% ASME SB443.4 NiCr22Mo9Nb
    Inconel 702 26 HRC 19% 5550
    Inconel 706 40 HRC 11% AMS 5702
    Inconel 718 42 HRC 10% 5383 NiCr19Fe19NbMo
    Inconel 718 DA 44 HRC 9%
    Inconel 718 OP 38 HRC 12%
    Inconel 718 Plus 42 HRC 10%
    Inconel 720 43 HRC 9%
    Inconel 722 34 HRC 14% 5541 NiCr16FeTi
    Inconel 725 37 HRC 13%
    Inconel 783 34 HRC 14%
    Inconel MA754 29 HRC 17%
    Inconel X-750 32 HRC 15% 5542 NiCr16FeTi
    Inconel X-751 35 HRC 14%
    M-252 46 HRC 5% 5551 G-NiCr19Co
    Monel 400 70 HRB 45% 4544 NiCu30Fe
    Monel K500 88 HRB 35% 4676 NiCu30Al
    Monel R405 80 HRB 45% 4674
    MP35N 28 HRC 18%
    Multimet N-155 27 HRC 18% 5768
    Multimet N-156 26 HRC 19%
    Nimonic 105 34 HRC 14% NiCo20Cr15MoAlTi
    Nimonic 75 90 HRB 17% NiCr20Ti
    Nimonic 80A 38 HRC 12% NiCr20TiAl
    Nimonic 90 28 HRC 10% NiCr20Co18Ti
    Nimonic 901 36 HRC 13% 5660, 5661 NiCr15MoTi
    Nimonic C263 28 HRC 18% NiCr20CoMoTi
    Nimonic PK33 38 HRC 12% NiCr20Co16MoTi
    René 41 36 HRC 15% 5712, 5713 NiCr19Co11MoTi
    S 590 27 HRC 18% 5533 X40CoCrNi2020
    Udimet 520 40 HRC 11%
    Udimet 718 42 HRC 10% 5383 NiCr19Fe19NbMo
    Waspaloy® 38 HRC 12% 5544 NiCr20Co14MoTi

    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.

    Material Hardness Machinability SAE DIN
    Stellite 151 46 HRC 6%
    Stellite 21 36 HRC 17%
    Stellite 25 (L605) 38 HRC 12% 5759 CoCr20W15Ni
    Stellite 31 (X40) 43 HRC 6% ASTM A567 CoCr25NiW
    Stellite 6 32 HRC 19%

    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.

    Material Hardness Machinability SAE DIN
    20CB-3 217 HB 45% ASTM B463
    A-286 250 HB 40% ASTM 368 X5NiCrTi2515
    Discaloy 16 290 HB 40% 5725
    Discaloy 24 280 HB 40% ASTM A638
    Incoloy 800 184 HB 50% ASME SB409 X10NiCrAlTi3220
    Incoloy 801 180 HB 50% 5552 G.X50CrNi3030
    Incoloy 802 180 HB 50%
    Incoloy DS 180 HB 50% X12NiCrSi3616
    Marval 18 470 HB 25%
    Udimet B-250 470 HB 25%
    Udimet B-300 470 HB 25%
    W-545 280 HB 40% AlSl:665

    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).