SURFACE HARDENING METHODS

OF STEAM TURBINE BLADES

Zoran Karastojković1, Momo Dereta2, Milesa Srećković3, Nikola Bajić4,

Zoran Janjušević5, Dragan Stojiljković6

Technical college, 11070 Novi Beograd, Blvd. Dr Zorana Đinđića 152a,

Vostok 7 –Ros-Sorab, 22400 Ruma, Karađorđeva 88,

Faculty of electrical engineering, 11000 Belgrade, blvd. kralja Aleksandra 73,

IHIS Technoexperts 11080 Belgrade, Batajnički put 23,

Institute for technology of nuclear and other mineral raw materials, 11000 Belgrade, Franchѐt d'Esperey 86,

SPIT, 16254 Bošnjace,

Every thermopower plant represents

a pretty complex system.

• It is evident that for safe working of the entire system many

elements/components are responsible.

• No dubt that one of the main machine component in every

thermopower plant are turbine blades, so they are often the

limiting component of a turbine and the entire power system.

• Their working life in a thermopower plant is over 100000h.

BLADES IN THERMOPOWER TURBINES

Some shapes&sizes of turbine blades

from thermopower plant

ABOUT BLADE/ELISE IMPORTANCE

According to some qualified opinions, the state of the art in producing of blade/elise has had the crucial effect on achieving the dominance of Great Britain in air during the II World war againt the Germans, and finaly for victory.

САМО НЕК СЕ ЗНА: НЕГДЕ КРАЈЕМ 70-ГОДИНА ПРОШЛОГ ВЕКА ЈЕДАН НАШ ИСТРАЖИВАЧ ИЗ ВИНЧЕ ЈЕ У ЕНГЛЕСКОЈ ДОКТОРИРАО НА ЛИВЕЊУ ОВАКВИХ ЛОПАТИЦА - АЛИ НАМА ЈЕ ТАДА, КАО ДРУШТВУ, БИО

ПРЕЧИ ЗАКОН О УДРУЖЕНОМ РАДУ...

НАСТАВАК СА ПРЕТХОДНОГ СЛАЈДА

...али зато ми за Икеју производимо

оклагије и кухињске даске!

MATERIALS USED FOR TURBINE BLADES

There is always a need for better materials, for achieving the

heavy servicing performances, so much researching in the field

of alloys and manufacturing techniques were done.

Here will be discussed next materials:

stainless steels, superalloys, titanium alloys and some ceramics

matrix composites.

Table 1. Stainless steels used for steam power

turbine blades in two standardsEN ГОСТ

Martensitic stainless steels

X12CrNiMoV12-3 1.4938; 1.4939 11ХН2В2Мф;

Х12Н2ВМф

X12Cr13 1.408 12X13;

X20Cr13 1.4021 20X13

X22CrMoV12-2 1.4922; 1.4293 20Х12ВНМф

X4CrNiMo16-5-1 1.4418 -

Precipitation hardening stainless steels

X5CrNiCuNb16-4 1.4542 -

X5CrNiCuNb14-5 1.4549 -

X5CrNiCuNb15-3 - -

SUPERALLOYS

Superalloys were developed in the 1940s.

Modern turbine blades often use nickel-based

superalloys that incorporate chromium, cobalt, and

rhenium, microalloyed with boron (<0,018mas%).

Another group of superalloys is on cobalt base.

The only one disadvantage of cobalt base alloys is in

high cost of this metal.

Turbine blades from superalloys are able

for directional solidification casting process

TITANIUM ALLOYS

EN ГОСТ

Titanium alpha near alpha alloy

Ti5Al2,5Sn 3.7114 -

Ti6Al2Sn4Zr2Mo 3.7155 ВТ3

Ti8Al1Mo1V - -

Titanium alpha-beta alloy

Ti6Al4V 3.7165 ВТ6

Ti6Al8V2Sn 3.7175 -

Ti6Al2Sn4Zr6Mo - -

Titanium beta-near beta alloy (not yet standardized)

Ti10V2Fe3Al - -

Ti13V11Cr3Al - -

Ti15V3Cr3Al3Sn - -

Scetch of some constitutional diagram

of titanium alloys

CERAMIC MATRIX COMPOSITES

In ceramic matrix composites (CMC) fibers are embedded

in a ceramic matrix, and such material are developed

mainly in use for gas turbine blades.

The main advantage of CMCs over conventional

superalloys is their light weight and high temperature

capability.

silicon base composites is consisting of silicon matrix

reinforced by silicon carbide fibers, which withstand

operating temperatures 100°-150°C higher than nickel

superalloys.

WHAT ARE REASONS FOR SURFACE HARDENING

OF BLADES

USABLE HARDENING METHODS FOR

TURBINE BLADES

Different methods for hardening of steam turbine blade

surface were developed&applied.

Some of them may be done at „new“ blades or partially

damaged.

The most known groups of usable methods for blade

hardening are: surface quenching, coating deposition by

chemical or electrochemical methods, brazing, hard-

surfacing (cladding) and shot peening („bombardment“).

Surface quenching

The outcomming part of a turbine blade some

producers treated by induction quenching.

If the parrent metal (with pearlitic-carbide

structure) shows hardness in range 260±20HV;

After induction hardnening (with tempered

martensitic structure) the hardness values usually

are in range 440±20HV.

(Electro)chemical depositionThere are two principal groups of deposition:

electrochemical (galvanic) and chemical (electroless)

method. As could be seen the uniformity of coat

thickness is superior at chemical deposition.

In chemical method is possible to

deposit not only metals but alloys.

One example of using chemical coating from Ni-P-B

alloy for protection a steam turbine blade

(martensitic steel ≈270HV) is provided in

termopower plant „Kolubara“, Serbia, by this autor.

The thickness of such uniform coatings in this

example was in range 20-25µm, with hardness

values about 500-540HV.

TIG cladding

The relatively large amount of heat input during

TIG welding/cladding is needed for melting of hard

(and heavy melted) materials,

Such great heating is undesirable when strong

tolerances of treated machine component must

remain.

This method is rare applicable for large surfaces.

Electrospark cladding

Electrospark deposition (ESD) is a pulsed micro-

welding process.

Electrospark deposition is used for small scale and

precision repair of high value worn or misfabricated

components.

For this method one could find terms as "spark

hardening", "electrospark toughening" or "electrospark

alloying".

Partially damaged blade may be repaired by deposition of

hard and tough coating by using a electrospark method, in

Fig. 6. is shown result when tungstencarbide (WC) is used

only at the turbine edge.

Brazing

Brazing as a production method, generally, is

disregarded.

But, brazing technology offers some specifics which

may be successfully applied for joining of a thin film

(layer) over a blade surface. Such film is made of a

corrosion resistant material, which also posses

antiwearing property.

Handling during brazing of such film needs pretty

skilled workers.

Laser cladding

Laser remelting in last decade became widely accepted

method for welding or cladding.

Cladding overlays b) and microstructure of laser

cladded and parent material c)

Plasma cladding

Plasma cladding is an able method for (re)melting

of material to be deposited, but this material

rather should be in a wire shape because the

powder is easily blown up.

Heating up of a parent metal in this method is

significant.

Shot peening

Shot peening is pretty simple cold working process,

similar to sandblasting: each (hard) particle

functions as a ball-peen hammer, producing

compressive residual stress layer, after that

mechanical properties of metals are modified.

The shot is used as round metallic, glass, wood, or

ceramic particles, with force sufficient to create

plastic deformation and then strengthening.

For shot peening of turbine-jet blades the most

valuable material are almond shells.

Shot peening in action is shown in Figs.

8a, b) and treated blades in Fig. 8c).

CONCLUSION

Steam turbine blades are exposed to heavy mechanical

loads at the moisture atmosphere.

So, their surfaces undergo to wearing and corrosion or

cavitation atack.

For satisfying such demands the steam turbine blades

should be manufactured from a qualitative material,

but some areas of blades need further hardening or

strengthening.

Here are shown contemporary methods for hardening

of steam turbine blades, which are applicable in

industrial circumstances.

ПОУКА (ИЛИ ЗАКЉУЧАК) НА СПРСКОМ:

Замена турбинских лопатица у једној

термоелектрани, у доба санкција, коштала је од

3-5 000000 марака.

Због блокаде земље оформљена је екипа људи

из: једне наше термоелектране, Машинског

факултета у Београду, Института за испитивање

материјала из Београда и Војно-техничког

института из Жаркова, да се такве лопатице (од

мартензитног челика) направе код нас.

Када је направљена прва лопатица, само по

геометрији, поносни професор је исту однео

неком из врха ЕПС-а.

Одговор је био: Ко вам је то рекао да радите?

ПИТАЊЕ ЗА ЖИКИНУ ШАРЕНИЦУ:

ДА ЛИ ЈЕ БОЉЕ ПРОИЗВОДИТИ

- ЈАКО СКУПЕ ЛОПАТИЦЕ ТУРБИНЕ ИЛИ

- ОКЛАГИЈЕ?

THANKS.

e-mail:

zoran.karastojkovic@gmail.com