The Development of WindTurbine Bolt Steels at

China Steel Corp."l A find power, a renewable energy source, is soaringV V in the 21st century. World wind generation capac-

ity doubled about every three years between 2000 and

A series of steels designed to meet the

rigid strength, toughness and hardenability

requirements of wind turbine bolts have

been developed. This work examines the

specific heat-treatment parameters of each

steel, including temper embrittlement, tensile

mechanisms and quench sensibility.

5.8, 8.8, 10.9 and 12.9. High-strength steel bolts have aproperty class of 8.8 or above, and wind turbine boltshave a property class of 10.9 or above. Materials of prop-erty classes 8.8 and above must have sufficient harden-ability to ensure the structure consists of approximately90% martensite in the core of the threaded sectionsfor the fasteners in the "as-hardened" condition beforetempering.3-6

Wind turbine bolts have become the newest prod-uct of the Taiwanese fastener industry in this decade.SCM435 (AISI 4135) was the material for M24-M27bolts in production, SCM440 (AISI 4140) for M30-M36bolts. The nominal diameter of a metric bolt is theouter diameter of the thread; M36 means 36 mm. Endusers have been dissatisfied with the inhomogeneous

Shou-Chi Lin (left) and Jui-Fan Tu (right), China Steel Corp., Hsiao Kang, Kaohsiung, Republic ofChina [Taiwan) (110643@mail.csc.com.tw, 157800@mail.csc.com.tw)

.•(a)

It &I 17 ., 15 14 U U

) •• • 1 •

a~~ \a"""'"DCulI:I'lO .••• .,a lo9CD'C1'd1.nlla--a~stlolttfl'l1ltwo-a RotOfIWlIh'5bIW

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c::I $hod: .•••••

C'fl/Wmg

r::J &'."1!I-CVINIl)aI$

1:1----(b)

Wind turbine bolts (a), nacelle and blades (b).

Authors

2006. The World Wind Energy Associationforecasted that, by 2010, more than 200 GWofcapacity would have been installed worldwide,another impressive 30% yearly growth ratefrom 2006 to 2010.

A wind turbine is a rotary device that extractsenergy from the wind. Horizontal-axis windturbines, the most popular ones, generally con-sist of a foundation, tower, nacelle and blades.With the growing worldwide emergence ofwind farms in recent years, wind turbine boltshave become the newly promising product ofthe Taiwanese fastener industry. Wind turbinebolts for the nacelle and blades, as with bolts forused in aerospace, are of the highest standard(Figure 1). Meeting the rigid strength, tough-ness and hardenability requirements for thewind turbine bolt steels is a challenge.I-2

Screws and bolts are made from a wide rangeof materials, steel being the most common andin many varieties. ISO 898.1 specification is theprimary specification used in wind turbine boltproduction. Some other specific requirementsmay be negotiated between buyers and suppli-ers. Tables 1 and 2 show the requirements ofmaterial, chemical composition, mechanicaland physical properties in ISO 898.1. Steelbolts usually have a hexagonal head with anISO strength rating (called property class)

stamped on thehead. The prop-erty classes mostoften used are

118 • Iron & Steel Technology

Material and ChemicalComposition in ISO 898.1Chemical composition limits

(cast analysis, %)Property classMaterial andheat treatment

......~_.~._--_.C p 5 B

Tempering temperature(OCmin.)

".,--_ .._- _._--------_. ~.------.._---- -- ._-

-- ._ ..... __ .....__ ..._-----------------10.9

12.9

Alloy steel quenched and tempered

Alloy steel quenched and tempered

0.2010.55 0.025 max. 0.025 max. 0.003 max.

0.3010.55 0.025 max. 0.025 max. 0.003 max.

425

425____________ ••••_"r __ . -..,..-- _ ..~ - --_ ..

Mechanical and Physical Properties of Bolts, Screws and Studs in ISO 898.1

Property class

10.9

Tensile strength(MPa)

1,040

Stress at 0.2%elongation (MPa)

940

Elongation(%)

9

Reduction ofarea (%)

48

. Rockwell hardness(HRC)

32-39

Impact strength(J, at -20°C)

27

12.9 1,220 1,100 8 44 39-44---------_._. __ ..- -- --~ ~.-

Heat-Treatment EvaluationTesting - Hardening of steelconsists of heating the metal

AI

0.023

0.032

Mo

0.19

0.19

Experimental ProcedureThe Jominy End-Quench Test - The Jominy end-quench test is the standard method to determinehardenability. The cylindrical specimen is heated to theaustenizing temperature. One end of the bar is cooledby a water jet in the recirculating water tank. Since oneend is quenched and the other is at room temperature,the cooling rate varies along the length of the bar. Mtercooling, the specimen is ground and hardness valuesare measured along the length using the Rockwell hard-ness testers. The hardness data are plotted vs. length togenerate hardenability curves (Figure 2). The fartheraway from the quenched end the hardness extends, thehigher the hardenability.The hardenability of ferrous alloys (i.e., steels) is

a function of the carbon content and other alloyingelements and the grain size of the austenite. The rateof cooling significantly affects the microstructure ofsteel. When a thick section of steel is quenched, thecooling rate varies with the distance from the surface.Hardenability refers to the depth to which martensiteis formed.8-9Two new medium-carbon Cr-Mo alloy steels, SCM449

and SCM453, were designed for M48 and M56 bolts,with adequate addition of hardening elements carbon,chromium and molybnedem to increase hardenability.Inclusion content and distribution, especially phospho-rus and sulfur, had a great influence on ductility andimpact energy, and were set to the minimum levels of the

mill. Table 3 shows the compo-sition ofSCM449 and SCM453.Jominy end-quenched testpieces were sampled from hot-forged steels to evaluate hard-enability of the steels .

1.01

1.00

0.009

0.008.._--_. ----------

..._-- ---------_.0.014

0.015

Jominy distance

0.77

0.770.24

0.240.49

0.53-- ..__ ...__ .. -- - -_ ..._-_ .._._._-----------

"==""support

SCM449

SCM453

ChemicalComposition of SCM449 and SCM453 (in wt. %)Steel C Si Mn P S Cr

The Jominy end-quench test.

mechanical properties of the M42 bolt with SCM440since 2009, which had unstable and insufficient hard-ness and strength readings. Mass effect caused the.instability in M42 bolts and above. M48 and M56 boltsare being developed in 2011-2012, with new bolt steelsplanned for the future. Some quench cracks, longitudi-nal cracks along the bolt body, occurred in production.The causes of these quench cracks are examined in thisstudy.7

October 2011 .• 119

Jominy end-quench test result of SCM440. SCM449 andSCM453.

1Ii!IZI1II _1----S-C-M-4-40-....---SC-M-4-4-9--.--S-C-M-4-S-31

II 13 15 20 25 30 35 40 45 50

Distance from Queching End ( mm )1.5 3

65

60

_ 55

~ 50

::. 45~= 40'E•• 35:c

30

25

20

Figure 3

Cl:l...:J-ro...Cl:lC.ECl:lI-

.{~:~:\i::.....Martensite --

TimeQuench and temper sequence.

to a high temperature (austenitizing) for a specifiedtime to complete transformation to austenite and diffu-sion of constituents, and then cooling in a quenchingmedium that produces the desired microstructure andas-quenched hardness. Quenching is necessary to sup-press the normal breakdown of austenite into ferriteand cementite, and to cause a partial decomposition atsuch a low temperature to produce martensite.To obtain this, steel requires a critical cooling velocity,

which is greatly reduced by the presence of alloying ele-ments, causing hardening and mild quenching (e.g., oiland hardening steels). This hardening treatment is mostoften followed by a lower-temperature heating process(tempering) to relieve stress and finalize the requiredmicrostructure to achieve the necessary physical proper-ties. This sequence is illustrated in Figure 3.The heat-treating condition for SCM440 was austen-

tizing at 840°C and tempering at 540°C. Heat-treatmenttesting evaluation was set for austentizing at 840°C andtempering at 200-700°C for SCM449 and SCM453.Several mechanical tests - tensile, hardness and impacttests - were performed on heat-treated test pieces.Microstructures of the tensile, hardness and impact

test pieces were observed under scanning electron andtransmission electron microscopes (SEM and TEM).Fractured surfaces of tensile and impact test pieces wereobserved under SEM.

Quench Sensibility Test - When steel is quenched, thevolume changes of austenite transformation occur veryrapidly and unevenly throughout the specimen. Stressesare set up, which may cause the metal either to distortor crack, if the ductility is insufficient for plastic flowto occur. Such cracks may occur some time after thequenching or in the early stages of tempering. 10-11Quench cracks are likely to occur due to:

• Excessive cooling rates during the quench (quenchseverity).

• Nonuniform fluid flow or contamination ofquenchant.

• Nonuniform heating or cooling or localizedover-heating.

• Prior steel structure.• Improper design of product.

120 • Iron & Steel Technology

A quench sensibility test was performed to access thequench sensibility of the steels. Hot-forged test pieces,r/J 50 SCM449 and r/J 60 SCM453, were used to performthe test. Test pieces after austentizing were quenchedto agitating 80°C oil and 20°C water, the most popularquenching media, in ordinary and specific ways. A jetwater quench, which poured water at one end of thetest piece, was tested. An oil-combined quench wasperformed using two SCM449 and SCM453 test pieces,which were fixed together with wire and quenched intoa no-agitation oil tank after heating. Each test piece wasexamined after quenching to see if quench cracking waspresent.

Results and DiscussionThe Hardenability of the Steels - Jominy end-quenchedtest pieces were sampled from hot-forged SCM449 andSCM453 steels. Test results of SCM440, SCM449 andSCM453 are shown in Figure 4. SCM453 had the besthardenability, SCM449 the second-best and SCM440the least hardenability. Each hardness reading for dif-ferent distances from the quenching end representeda specific cooling rate. At a certain cooling rate, forexample atJ7 (13 mm or 7/16 inch from the quenchingend), the hardness ofSCM440 was HRC50, SCM449 wasHRC57 and SCM453 was HRC60. The farther away fromthe quenched end the hardness extends, the higher thehardenability. For hardness above HRC50, the readingfor SCM440 was atJ7, SCM449 was atJIO and SCM453was at JII. With a substantial increase of hardenability,SCM449 and SCM453 steels met the uniform micro-structure requirements of M48 and M56 bolts, withmore than 90% martensite in the core of the threadedsections for the fasteners in the "as-hardened" conditionbefore tempering.

Heat-Treatment Evaluation - Heat-treatment testingevaluation was arranged to be austentizing at 840°Cand tempering between 520 and 620°C for SCM449 andSCM453. Evaluation results of mechanical propertiesare shown in Tables 4 and 5. As tempering temperatureincreased, strength (yield strength, tensile strength,hardness) decreased and ductily (percentage of elon-gation and reduction of area) increased. At the same

1IE.1mDI _Tempered Hardness of SCM44 and SCM453 (in HRC)Steel 200.C 300.C 400.C SOO.C 600.C 700.C

SCM449

SCM453

57

58

5253

4950

43

4535

38

26

27

---Mechanical Properties of SCM449 and SCM453Yield strength Tensile strength Elongation Reduction of Impact strength

Steel (MPa) (MPa] (%) area (%) (J, -40.C)

ISO 898.1 940i 1,040i 9i 48i 27 Ji

SCM449 - 580.C 1,020 1,140 18.5 54.45 41.9

SCM449 - 600. C 980 1,099 18.12 54.95 58.8

SCM453 - 600.C 1,007 1,131 18.32 52.92 55.8

SCM453 - 620.C 954 1,089 19.88 55.91 42.2

(a) (b)

(c) (d)

SEM microstructure of SCM449 at 500 0C (a) and 600 0C (bl. and SCM453 at 500 0C (c) and 600 0C (d).

October 2011 • 121

tempering temperature, SCM449 had lower strengthand higher ductily than SCM453. For 10.9 bolts, the tar-get hardness reading should be HRC35-37; lower hard-ness would result in insufficient strength, and higherhardness would result in insufficient ductility.

Austenizing temperature is 840°C for both hypo-eutectoid steels, the same as SCM440. Adequate temp-ering condition for SCM449 should be 580-600°C,and 600-620°C for SCM453 after quenching. All themechanical requirements - strength, elongation andimpact energy - could be met by adequate heat-treat-ing operation.

Figure 5 shows that the SEM microstructures ofSCM449 and SCM453, tempered at 500°C and 600°C,were more than 90% tempered martensite. Precipitatedcarbide was more obvious when observed on the 600°Ctest pieces. TEM observation of the 600°C test pieces areshown in Figure 6, with 10-60 nm carbides precipitatedalong grain boundaries and within grains under hightempering temperature. Some recrystalization mightoccur, and some new grains were noted without disloca-tion inside.

Fractured surfaces of tensile test pieces were exam-ined under SEM. Cracks were forming in an area in

the center, 1-3 mm in dia~eter, which then grewoutward and resulted in a fracture shown in Figure 7a.Minor cracks are shown along with inclusions in thesteels in cross-section observation of fractured surface(Figure 7b). The inclusions were iden tified as MnS byenergy-dispersive x-ray spectroscopy (Figure 7c).

Results of Quench Sensibility Test - In the generalwater and oil quench procedures, no quench crackswere observed. In the jet water quench, in which waterwas poured at one end of the test piece, circular endcracks appeared on the quenching ends of both steels.These circular quench cracks were due to excessive cool-ing rates during the jet quench, as shown in Figure 8a.

In the oil-combined quench, the combined test pieceswere heated and then quenched into a no-agitation oiltank. A longitudinal crack appeared on one side of theSCM453 test piece (Figure 8b), while no crack appearedon the SCM449 test piece. The combined test pieceshad an uneven cooling: the cooling rate of the contactside was slow while the non-contact side was fast. Thedifference of cooling induced a longitudinal stress fieldand a quench crack appeared. The quench cracks expe-rienced in end-user plants were longitudinal quench

(a) (b)

TEM microstructure of SCM449 (a) and SCM453 (bl. tempered at 600 °C.

(c)

00 (~SEM fractured surface of SCM449 (a), minor cracks, along with inclusion (b) and MnS (c).

122 + Iron <51 Steel Technology

(a)

Circular quench cracks (a) and a longitudinal quench crack (b).

(b)

cracks on the bolts. Poor racking of the parts prior tothe quench or nonuniform fluid flow around the part inthe oil quench tank is likely the cause of this particularquench crack.

Manufacture of Bolts at End-User's Plant - TheSCM449 and SCM453 steels were vacuum-melted, ingot-cast, hot-forged, spheroidizing-annealed and machinedto the dimension for end-user trial. Sample steelsSCM449 and SCM453 were sent to the customer for M48and M56 bolts manufacturing. Head forging, threadrolling and heat-treating practices were smoothly con-ducted. Tables 6 and 7 list the mechanical and physicalproperties of the bolts. Hardness distribution of thecross-section after quenching had a very uniform result.Strength, elongation and impact energy were good, but

1IIi!tmII _Tempered Hardness of SCM449 and SCM453 onthe M48 and M56 bolts (in HRC)

the percentage reduction of area was below the 48%requirement. Testing temperature of the impact test was-40°C, stricter than -20°C by specification.

The types of tensile test pieces used could lead to thedifference in reduction of area readings between the laband the plant. In ISO 898.1 specification, a machinedtest piece from large bolts should be used for tensiletest, as shown in Figure 9. For M48 bolts, a machinedtest piece with 36 mm-\jf should be tested. A 20 mm-\jfmachined test piece was used to perform the tensile testfor large bolts in the plant because of the limited capac-ity of the old 50-ton universal testing machine. Also, labtest pieces sampled from 30-mm bar after forging, withhigher reduction ratios, had better readings than testpieces of bolts from 55-mm bar.The SCM449 and SCM453 steels were accepted by

the end users after the first test. A second test wasthen arranged. During the second bolt test, 36 mm-\jfmachined test pieces were tested on a 200-ton universaltesting machine, and all the properties met the mechan-ical requirements, with an acceptable 52% RA.

---------Mechanical Properties of SCM449 on the M48 Bolts

Yield Tensile Reductionstrength strength Elongation of area

Test (MPaJ (MPaJ (%J (%J

ISO 898.1 940i 1.040i 9i 48i

SCM449-M48 1.048 1.145 12.9 44.2

M48 second test 1.023 1.124 14.8 52.2 45.7

Reduction of Area (RA) Test - It was reported thatthe percent reduction of area was directly related tothe microstructure. Different microstructures, such asferrite and pearlite, ferrite and cementite, bainite andmartensite, had a certain value range of percent reduc-tion of area in the tensile result. From the database ofthe National Institute of Material Science in Japan, thevalue percent reduction of area of ferrite and pearl-ite was between 45 and 70%, bainite was between 62

and 82%, tempered martensite was between50 and 65% and ultrafine-grained steel wasbetween 45 and 70%. For a ferrite and cement-ite microstructure, percent reduction of areareading decreased as the volume of cementiteincreased. As shown in Figure 10, the volumeof cementite increased from 0.1 to 6%, and thepercent reduction of area reading decreasedfrom 80 to 60%.12-13

Due to the incident in the bolt plant, areduction of area test was performed to realizethe tensile mechanism. Test pieces of SCM449were pulled on several different stages to seethe progress of the tensile test. Compared with

30.7

27 Ji

Impact(J, -40°C)

35.7

36.2

35.1

36.2

36.3

36.1

36.3

35.9

37.2

38.7

37

37.5

Center

58.5

57.5

61.5

61

As-quenchedTest

SCM 449-M48

SCM 453-M56

SCM 449-M56

SCM 453-M48

October 2011 • 123

--------

(b)

1,2001,000800600

Tensile strength (MPa)

(a)

400

Ultrafine grained sleelsBainite" O.02C O.05C

.•• ~?,15Cn"

~D D~~~45C

F+P,S25C-S45C17- ~ ' •

Tempered martensite of S45C17

o

200

Volume fraction of cementite (%)

0.00.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

20.0

80.0

60.0

40.0

100.0

100

90

"#.80

'"70

~ 60'".!: 50ca

4013::J"0 30Q}

0::20

10

00

(a)

Key

Lt

s.

d nominal thread diameter

do diameter of machined test piece (do < d3min but, whenever poSSIble, do " 3 mm.)

b thread length (b ;, d)

L. original gauge length of machined test piece

- for determination of elongation: La = 5do or (5,65.jS;)

- for determination of reduction of area: La " 3 do

Le length of straight portion of machined test piece (La + do)

I, totallengfh of machined test piece (Lc + 2 r + b)

So cross.sectional area of machined test piece before tensile test

r fillet radius (r " 4 mm)

1IIIaII _

(b)A machined test piece in ISO 898.1 specification (a) and amachined test piece from stud (b).

Percent reduction of area with different microstructures(a) and reduction of area under different volume of secondphase (b).

Testpieceswith different elongation % (a)and metallurgical sample of 17% test piece (b).

(b)

the reduction of area, elongation readings were mucheasier to control in a universal tester. From the stress-strain curve of SCM449, the load reached a maximumvalue of around 7% strain, and then necking began. Testpieces with different elongation readings of 10, 12, 14,

(a)

16, 17 and 18% were performed and then stopped andreleased for further investigation. The reduction of areareadings linearly increased with elongation readingsbetween 10 and 18% elongations. The 18% test piecewas broken at once on the machine upon load release.

All elongation-RA test pieceswere examined by x-ray radio-graphic inspection, to see ifany flaw developed duringdifferent tensile elongationstage. Radiographic inspec-tion is a nondestructive test-ing technique used to evalu-ate objects and componentsfor signs of flaws which couldinterfere with their function.It is accomplished with the useof radiographs, images gener-ated by bombarding the objectunder inspection with radia-tion. X-ray and gamma rayradiographic inspection arethe two most common forms

124 + Iron & Steel Technology

of this inspection technique. China Steel Corp. has itsown nondestructive inspection facilities for in-houseinspections. No flawswere observed in all elongation-RAtest pieces, even in the 17% elongation test piece shownin Figure 11.14The basic steps of ductile fracture, void formation,

void coalescence (also known as crack formation),crack propagation and failure often result in a cup-and-cone-shaped failure surface. For a high-strength mate-rial, there are three steps in a fracture process: crackinitiation, crack propagation and final fracture. Thebolt material SCM449, with 18% elongation and goodtoughness, had a very long crack initiation step, whilecrack propagation and fracture followed and developedsoon after.

SummaryA series of steels for M42, M48, M56 and M64 bolts,with careful alloy design and inclusion control, havebeen developed. Steels SCM449 and SCM453, withadequate hardening elements and inclusion control,could meet the heat-treatment requirements of M48and M56 bolts. Hardness distribution of the cross-section after quenching had a very uniform and steadyresult, and ensured 90% martensite microstructurerequirement. Austenizing temperature is 840°C forboth steels. Tempering condition for SCM449 is 580-600°C, and 600-620°C for SCM453 after quenching. Allthe mechanical requirements, strength elongation andimpact energy should be met by proper heat-treatingoperation. M48 and M56 bolt samples, through a realproduction line at the end user's facility, were made inthis project. Satisfactory trial results were achieved andthe first order shipped to customers in April 2011 forM48 and M56 wind turbine bolts.

References1. "World Wind Energy Report 2009." World Wind Energy

Association.

2. http://www.gepower.com/busi nesses/ge_wind_energy/en/index.htm.

3. International Standard ISO 898.1. "Mechanical Propertiesof Fasteners Made of Carbon Steel and Alloy Steel. Part 1: Bolts,Screws and Studs With Specified Property ClassesCoarse Threadand Fine-Pitch Thread."

4. ASTM A354-07a, "Standard Specification for Quenched andTempered Alloy Steel Bolts, Studs and Other Externally ThreadedFasteners...

5. ASTM A490M-09a, "Standard Specification for High-Strength Steel Bolts, Classes 10.9 and 10.9.3, for Structural SteelJoints (Metric]."

6. ASTM A574M-08, "Standard Specification for Alloy SteelSocket-Head Cap Screws (Metric)."

7. w.e. Yeng, China Steel Corp. T21 Report B-980512, May2009.

8. R.e. Sharma, "Principles of Heat Treatment of Steel,"Technology& Engineering, 1996.

9. G.E. Totten, e.E. Bates and N.A. Clinton, Handbook ofOuenchants and Ouenching Technology, ASM International,Materials Park, Ohio, 1993.

10. R.R. Blackwood, L.M. Jarvis, D.G. Hoffman and G.E. Totten,"Conditions Leading to Quench Cracking Other than Severity ofQuench," 18th Heat Treating Society Conference Proceedings,eds., H. Walton and R. Wallis, ASM International, Materials Park,Oh~, 1998, pp. 575-585.

11. K. Arimoto, F. Ikuta, T. Horino, S. Tamura, M. Narazaki andY.Mikita, "Preliminary Study to Identify Criterion for Quench CrackPrevention by Computer Simulation," Transactions of Materialsand Heat Treatment, Vol. 25, NO.5, 2004, pp. 486-493.

12. S. Torizuka, E. Muramatsua and S.V.S. Narayana Murty,"Microstructure Evolution and Strength-Reduction in Area Balanceof Ultrafine-Grained Steels Processed by Warm Caliber Rolling,"Scripta Materialia, Volume 55, Issue 8, October 2006, pp.751-754.

13. S. Torizuka and E. Muramatsua, "Microstructure Evolutionand Strength-Reduction in Area Balance of Ultrafine-GrainedSteels Processed by Warm Caliber Rolling," CAMP-ISU, Vol. 18,2005, p. 1754.

14. China Steel Corp., T42 NDE-RT Report, 99-RT-T18-003,May 2009. . +

This paper was presented at AISTech2011 - The Iron & Steel Technology Conference and Exposition,Indianapolis, Ind., and published in the Conference Proceedings.

if'IJA'f;,!ft:,

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October 2011 + 125