Turning Analysis and Improvement Measures of Several Superalloy Materials

(HY-industry technical centre)

ABSTRACT: With the rapid development of China’s machinery manufacturing technology, the requirements for the structure and technological properties of metal materials are getting higher and higher. The number of newly developed or imported engineering materials is increasing. Some materials with high temperature oxidation resistance, wear resistance, corrosion resistance, creep resistance and high strength are available. For example: GH4145(Inconel X750), GH901(Incoloy 901) , GH80A(Nimonic 80A), WR26, K403…. And so on. Now it has been used in steam turbine thermal power units, which has revolutionized the service life and energy-saving technology of steam turbine. Although the application of these new materials has created tremendous social and economic benefits. But because these materials contain a lot of alloy elements, cutting is very difficult.

Taking

GH4145(Inconel X750), GH901(Incoloy 901) , GH80A(Nimonic 80A)as examples, this paper introduces some of my analysis and experience: (1) Key words: superalloy, analysis and improvement measures (tool material, tool angle, cutting fluid).

I:Analysis

Main Chemical Compositions of Three Materials:

Alloy 901(Incoloy 901) Chemical composition:

% Ni Cu Al Fe Ti Co Mn S C Si P Cr Mo B
Min. 40.0 2.35 11.0 5.0 0.01
Max. 45.0 0.50 0.35 Rest 3.10 1.0 1.0 0.03 0.10 0.60 0.03 14.0 7.0 0.02

Inconel X750 Chemical composition:

Alloy % Ni Cr Fe Nb Co C Mn Si S Cu Al Ti
Inconel X750 Min. 14 5 0.7 0.4 2.25
Max. 70 17 9 1.2 1 0.08 1 0.5 0.01 0.5 1 2.75

Nimonic 80A Chemaical composition:

% Ni Cr Fe Mn Si P S Al Ti C Co B Zr Cu
Min. Balance 18 1.0 1.8
 Max.  21  3.0  1.0  1.0  0.02  0.015  1.8  2.7  0.1  2.0  0.008  0.15  0.2

It can be seen from the above chemical composition that GH901 belongs to iron-based superalloy, which is relatively cheap because of its nickel content of only 40%-45%. Its cutting performance is also better. It is similar to austenitic stainless steel. GH4145 and GH80A are nickel-based superalloys with over 65% nickel content. This kind of alloy is the most stable material with high temperature oxidation resistance at present. But at the same time, their cutting performance is very poor. The content of easy-to-cut elements (sulfur, phosphorus, lead and bismuth) is low or not.

I think the master who has experienced the processing of this kind of material will say, “More time is spent in the grinding room than in the cutting room”. Some free forgings have thick oxide skin and strong adhesion, uneven surface, and the first and second cutters are not round. Turning is more headache. If the workpiece rotates a few times, the tool will be seriously worn out, so that it can not be processed properly. Even lead to product scrap. Because of the high price of such materials, the cost of a product is hundreds of thousands of dollars. Business losses can be imagined.

Superalloy Why is it difficult to cut?

CNC machining1. Superalloy has high strength and obvious work hardening. When cutting, the plastic deformation is large, the cutting load is large and the cutting temperature is high. The unit cutting force of some nickel-based superalloys is more than 200% higher than that of medium carbon steel. The surface layer of the machined workpiece has high work hardening and residual stress. The degree of cold hardening can reach 250%-500%. The tool tip and boundary wear are very serious. The groove wear of the auxiliary flank is also very easy to occur.

2. Poor thermal conductivity: When cutting superalloy materials, plastic deformation consumes a lot of energy, and more than 90% of this energy is converted into heat energy. However, the thermal conductivity of such materials is poor and heat dissipation is difficult, so that high cutting heat is concentrated in the cutting area. The general temperature can reach 1000 degrees Celsius. It increases the speed of tool phase change wear and bond wear.

3. High content of strengthening elements. There is a tendency of dispersion strengthening, that is, when the cutting heat reaches a certain temperature, a large number of hard particles such as metal carbides and intermetallic compounds with strong abrasiveness are formed in the alloy. It has strong mechanical abrasion and wear effect on cutting tools.

From the above analysis, we can see several reasons for tool wear:

  • mechanical abrasion wear,

  • bond wear,

  • phase change wear,

  • diffusion wear,

  • oxidation wear.

Almost all of them occur when turning superalloys. It is not difficult to understand why the tool wears so fast.
In the past, YG8 or YG6 were commonly used in turning forgings of this kind of superalloy material.γ0 = 8° ~14°,α0 =6° ~10° ,Cutting speed V=20-35 min, feed F=0.2-0.35 m/r and cutting depth alpha p=3-7 mm are adopted for rough turning. Although tool durability is improved, the total production efficiency is still not high. A GH80A(Nimonic 80A)valve stem of about 400_can only be roughed up for 38 to 40 hours at the fastest speed. Far from meeting the needs of production.

YG3, YW1, YW2 for finishing car.

II: Improvement Measures:

After constantly exploring the rules and analyzing the reasons, a complete set of turning process is gradually summarized:

first, heat treatment is improved.

(1)Iron-based superalloy GH901(incoloy 901) was treated by solid solution and aging (heat preservation at 1070 ~1080 C)
2 hours, air-cooled) aging (1, heating to 780 ~790 for 4 hours, air-cooled, 2, heating to 710 for 24 hours, air-cooled)

(2)GH4145 (inconel X750)Solid Solution + Aging Treatment (Heating to 1100 +10, Holding for 2 hours, Air-cooling)
Aging (1) Heating to 845 +10. Insulation for 24 hours, furnace cooling less than 2 hours, 2, warming to 705 +10, holding for 20 hours, air cooling)

(3) GH80A(nimonic 80A) solid solution + aging treatment (warming to 1080 C for 8 hours, air cooling) aging

  • Heating to 700 C for 16 hours, air-cooled.

  • Warm up to 750 C for 4 hours, air-cooled.

Some improvements can be made in turning nickel-base superalloy steel from the following aspects:

  • Tool Material,

  • Tool Geometric Angle,

  • Use of high-power equipment,

  • Cutting parameters,

  • Cooling mode and coolant varieties.

A. Tool material. Rough Car: YG10H or YG8W, Precision Car: YG813, YG643. YG10N is a superfine grain alloy with good wear resistance, impact resistance and vibration resistance. Suitable for rough turning nickel base superalloy. YG8W: Abrasion resistance and allowable cutting speed are higher than YG8. It has good impact and vibration resistance. It is suitable for iron-based superalloy of rough turning. YG813: It has high heat resistance, high temperature toughness and anti-sticking knife, good versatility. Suitable for nickel-base superalloy for precision turning. YG643: It is a sub-fine grain alloy with high wear resistance, oxidation resistance, good adhesion resistance, good heat resistance and toughness. Suitable for finishing iron-based superalloy special profile, groove and thread of finishing car, high speed steel can be used: W2Mo9Cr4VCo8, W18Cr4V.

B. Improvement of Tool Geometry Angle:.

  • When forgings GH80A(nimonic 80A) and GH4145 (Inconel X750)of 0.1 roughing lathe are forged, the rake angle of the tool should be slightly smaller, and gamma γ0 = 10° ~18°.(For example, car casting superalloy steel should be smaller and gamma γ0 = 4° ~8°). Back angle alpha α0 =10° ~18° and rake face can be changed to 0.2MM flat edge. The edge should be sharp and the roughness of front and rear cutting edges should be grinded below Ra 1.6. Cutting tool grinding full circular arc chip breaking groove. The main deviation angle ranges from 65° ~70°. The inclination angle of the edge is from λs= -5° to – 8° . When rough turning GH901 iron-based superalloy steel, according to the cutting tool angle of processing austenitic stainless steel, gamma γ0 = 12° ~20° and alpha α0 =8° ~10°. The main deviation angle is 75. λs= -5° to – 8° Negative chamfer can be grinded in order to improve tool durability. Grinding tool workpiece surface to Ra1.6 below

  • Fine turningGH80A(nimonic 80A) and GH4145 (Inconel X750), with slightly larger front and rear angles of cutters: gamma γ0 = 20° ~25° and α0 =8° ~25°, in order to protect the edge strength, it is best to grind double rear angle. Grinding the cutting edge band from the flank. Front

    Negative chamfering must not be ground. In short, the cutting edge should be sharp, and the roughness of the front and back of each cutting edge should be ground to below Ra0.8. The main deviation angle should be 80° ~90°. The inclination angle of the edge is λs= 4° to -6°. When finishing car GH901, gamma γ0 = 15° ~20° and α0 =10° ~18°. The main deviation angle is 90°. λs= 0° to 5°The radius of tool arc R groove is 3 mm wide and 2 mm deep. In order to improve tool durability, chamfering of -5° can be grinded out. The tool workpieces are ground to below Ra 0.8.

C.Equipment selection: When rough cutting, the machine power must be high. Because the cutting force required for turning high temperature-alloy is 200% larger than that for turning medium carbon steel under the same conditions, in order to avoid contact between tool tip and oxide skin, turning round workpiece should be done once as possible when turning forgings. Therefore, the cutting force is greater.

D.Equipment selection: When rough cutting, the machine power must be high. Because the cutting force required for turning high temperature-alloy is 200% larger than that for turning medium carbon steel under the same conditions, in order to avoid contact between tool tip and oxide skin, turning round workpiece should be done once as possible when turning forgings. Therefore, the cutting force is greater.

  • When roughing, AP = 5-10mm_v = 40-60m/min_f = 0.18-0.35mm. Cutting depth is as large as possible to avoid the rough skin or the hardening layer left by the previous process. Cutting speed is reduced in intermittent cutting according to the actual situation.

  • When finishing, AP = 0.15-0.4 mm v = 60-80m/min_f = 0.12-0.18mm.The cutting speed of special profile, groove and thread of high-speed steel car is reduced according to the actual situation.

cutting fluid also known as cooling lubricant, it is used to reduce friction and cutting temperature in cutting process, so as to improve tool life, processing quality and production efficiency. Commonly used cutting fluids are cutting oil, emulsifying fluid and chemical cutting fluid.

E. Cooling methods and types of coolants:

  • T multi-functional emulsified oil), when used, the emulsified oil of 2%-3% concentration is mixed with soft water according to the requirement.

  • Solanedioic acid 7-10%+sodium nitrite 5%+triethanolamine 7-10%+boric acid 7-10%+glycerol, and add water.

High pressure cooling mode is adopted. The diameter of nozzle hole is 0.5-0.7 mm, the pressure is 1.5-3 MPa, and the flow rate is 0.2-0.5 liter/min. The nozzle should be as close as possible to the cutting edge.

Through the improvement of the above methods, the production efficiency of Superalloy steel in the mechanical processing workshop led by HY-industry is 43% higher than that without improvement. The rate of equipment damage is obviously reduced. Workers’earnings have risen sharply. The cost of cutting tools is only about 30% of the original cost. Technology is endless. As long as we dare to emancipate our minds, boldly explore the laws and carefully summarize them, any difficulties can be overcome. Shanghai HY Industry Co., Ltd. has become a customized unit of Shanghai Steam Turbine Co., Ltd., replacing some imported parts. This year, we have cooperated with Jiangsu Suanyang Precision Alloy Factory in the development of high temperature alloys for large aircraft manufacturing projects in China. Next year we will enter the conventional Peninsula nuclear project. How to overcome more cutting difficulties is an urgent problem for our mechanical processing technicians.

HY-industry is qualified  Nickelalloy&cobalt alloy materials supplier.

We have more than twenty years experience in kind of High temperature alloy production.

Monel 400,Stellite 6,Stellite 12,Incoloy 800ht,Nimonic 80A,Kovar,Invar 36,Inconel 625,Hastelloy C276, Incoloy 825 are mature productes of us.

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