CHAPTER-I

INTRODUCTION TO Cr-Mo- V STEELS

1.1 General

1.2 Hydro Cracker Reactor

1.3 Material Selection for Hydrocracker Reactor

References

----- - - -------------------------------

CHAPTER -1

INTRODUCTION TO Cr - Mo - V STEELS

1.1 General

Steel is arguably the world's most "advanced" material. It is a very versatile

material with a wide range of attractive propcrties, which can be produced at a

very competitive production cost. It has a diverse range of applications, and is

second only to concrete in its annual production tonnage. Steel is not a new

invention which leads to a common misperception that "everything is known

about steel" amongst those outside its field. On the contrary. research within this

field is probably the most challenging of all the material sciences. It is grounded

on decades of research and much fundamental theory arises from the study of

steel. Steel is generally defined as a ferrous alloy containing less than 2.0 \\1. %

C [I]. The complexity of steel arises with the introduction of further alloying

clements into the iron - carbon alloy system. The optimization of alloying content

in the iron - carbon alloy system, combined with different mechanical and hcat

treatments lead to immense opportunities for parameter variations and these arc

continuously being developed.

Through the last few decades a category of steels known as high strength steels

have undergone constant research. As a direct result, steels with yield strengths

greater than 1100 MPa combined with good toughness at low temperatures are

available such as SSAB's , Weldox 1100 containing 0.2 C, 1.4 Mn, 3 Ni and

minor additions of Cr, Mo, Si, Cu, Y, Nb, Ti, AI and B. Today, as new and greater

challenges are presented to designers and engineers, these steels are increasingly

employed in a diverse range of applications such as bridges, pipelines, submarines

and crancs. Constnlctions with lower structural thickness and weight, with the

same load bearing capabilities are often possible with the employment of such

steels.

In many of these applications it is essential to form strong joints that allow

transfer of load between the different steel components. Generally, welding is the

preferred joining method since it forms a continuous joint, it alleviates corrosion

problems often associated with fasteners and it offers greater beauty to the

application. In many circumstances, it is a structural requirement that the weld

metal has over-matching strength in comparison to the steel in order to avoid

design limitations. These requirements are possible to achieve under well­

controlled conditions, gas tungsten arc welding (GTAW). However as strength

levels increase it becomes more difficult to fulfil impact toughness requirements

with flexible and productive welding methods such as shielded metal arc welding

(SMAW), flux cored arc \\doing (FCA W). or sub- merged arc welding (SAW)

[2-3]. This project encompasses the SAW -AC technique with a view to

increasing impact toughness of steel weld metals while keeping yield strength

greater than 690 MPa (100 ksi or 105 Ib/in2).

All our the world, energy crisis is tremendous and particularly for India. which is

a developing country and there is 10 to 15% annual increment of energy

requirement. At present India rely mostly on thermal energy. One of the basic raw

material of thermal energy is petroleum product for this, the most important

equipment is hydrocracker reactor. This equipment uses Cr-Mo-V steel in its

fabrication, this metal is very complex and difficult to understand during welding

process. Therefore this project tries to evaluate the essential parameter, their

mutual interdependence and to find out their relationship.

2

1.2 Hydrocracker Reactor

Flexibility of operation is the key word in the petroleum refining industry today.

As the market demand have shifted from high to low sulfur fuels, and from fuel

oil to lighter products, the refineries have no other option but to place an added

emphasis on various alternative processes which are externally flexible and cost

effective. Coupled with deterioration in average crude quality, this has inevitably

led to increasing demand for residuum upgradation technology[ I, I 0].

There are three widely used conversion processes for upgradation of heavy end of

hydrocracker viz. Coking, Fluid Catalytic Cracking and Hydro cracking. While

the first two processes are based on carbon rejection technique, whereas last

process is based on breaking the C-C bond with simultaneous hydrogenation

processes are as under:

Table 1.1 Comparisons of Hydrogenation Processes[I,IO]

SR VARIABLES COKING FCC HC

NO.

I Operating Pressure Low Low High

2 Special metallurgy No No Yes

3 Operating Flexibility Low High High

4 On stream Factor Low Medium High

5 Investment Cost Low Low High

6 Feed preparation read No Yes Yes

7 Conversion Low Medium High

8 Product Quality Unstable Unstable Stable

9 Sulfur removal No No Yes

FCC = Fluid Catalytic CracklOg HC = Hydro crackIng

3

The major auvantages of Hyuro cracker over the other two alternatives are:

[1] Since Hydro cracking involves addition of hydrogen to the carbon atoms,

distillate yield is much higher as compared to carbon rejection processes.

[2] Conversion to middle distillates in the case of hydrogen cracking is more

than 80% against 40% in the case of catalytic cracking.

[3] Even though Catalyst cracking produces large quantities are far from

Specification. But the products from hydro cracker are stable and free

from sulfur and nitrogen compounds and hence no additive requirement

for improvement in storage stability.

[4] A salient feature of hydro cracking is its flexibility with respect to

Product pattern. Units are designed and operated to produce LPG,

Naphtha, Gasoline, Middle distillates, lube base stocks, Ethylene plant

feed and FCC feed.[ I, I 0]

1.2,) History of Hydro Cracking

Hydro cracking is one of the oldest hydrocarbon conversion processes and was

commercially developed by I.G. Farben industries in 1927 for processing of

lignite to gasoline. This technology was extensively used between 1935 and 1945.

In early 1930's, this technology was used in USA for upgrading lube oils, diesel

fuels and burning oils. These processes were primarily high-pressure processes. In

early 1940's, the important of hydro cracking came down due to development of

catalytic cracking processes and increased availability of middle east

cmdes[ 10,15].

By the end of 1950's, hydro cracking process have gained acceptability again due

to various factors. Improved catalysts were developed later which permitted

operation at relatively low pressure resulting in a reduction in the uemand of

4

heavy petroleum products as a result of conversion nf rail/road transport from

steam to diesel and use of nature gas in the place of heating oil coupled with

higher demand of gasoline placed the emphasis on the conversion of surplus fuel

oil to distillate products. These factors together with the availability of relatively

low cost hydrogen (from catalytic reforming) and advances made in mechanical

engineering especially in the field of reactor design pushed up the case of hydro

crackingll 0, 15).

Since then, the worldwide hydro cracking capacity has been increased steadily.

Not all these units were true hydro crackers as it defined today. Many of them had

the basic function of product and feedstock upgradation such as cycle oils,

thermal and Coker gas oils and other heavy gas oils. With the improvement in

catalyst, nowaday hydro cracker has a broad range of products ranging from

LPG to middle distillates, lube base stock, ethylene plant feed and FCC feed. It is

now particularly recognized as a premium technology for products of high quality

middle distillates demanded by market.

In advanced countries with high demand for gasoline, Catalytic cracking and

hydro cracking have been employed as team. The catalytic crackers take the more

easily cracked paraffin feed stock, while the hydro crackers use more automatic

distillates as feed. Even though the new Zeolite catalyst have helped in improving

gasoline yield and octane quality from catalytic crackers, it cannot crack the cycle

stock to extinction. Such stocks find difficult application, but form excellent

charge for hydro cracking. In addition to vacuum gas oil, cycle stocks etc., it is

also possible to process residual oil and reduced crude by hydro cracking. The

difference between the two types of processes is in the type of catalyst and

operating condition[ 10, 12J

5

1.2.2 Hydro Cracking

Hydro cracking processes has a wide range of application. The feed stocks can be

straight run gas oils, thermally gas oil, cycle oils, cocker gas oils, thermally

cracked stocks, solvent deasphalted residual oils, and straight run naphtha,

cracked naphtha, atmospheric residue or vacuum residue. Its products states

consists of LPG, MS, reformer feed, ATF, diesel fuels, heating oils, FCC or

ethylene plant feed, lube oil base stocks etc. Large number of commercial

processes and patents are available. In all these processes, hydro cracking as well

as hydro treatment are carried out generally in fixed bed catalytic reactors in the

presence of hydrogen under high pressure. Various types of catalysts such as

Cobalt-Molybdenum or Nickel- Tungsten supported on Silica-Alumina or Zeolite

are used in these processes. Depending upon the feed and the end products

required, the operating condition and catalyst for hydrocraking may vary. Fig. 1.1

shows a fabrication procedure for reactor pressure vessels[ I 0, 15].

Tandem SAW GTAWtSAW SlJrt

ln~m!1 o!emal

Fig. 1.1 A Fabrication procedure for reactor pressure vessels.

6

Common Hydro cracker configurations are[ 15]:

[I] Single stage hydro cracking

[II] Two stage hydro cracking

[III] Series flow hydro cracking

When the treating step is combined with the cracking reaction to occur in one

reactor, the process is called a SINGLE-STAGE PROCESS.

[I] Single stage hydro cracking[lO,15]

In this simplest of the hydrocracker configuration, the layout of the reactor

section generally resembles that of hydrotreating unit. This configuration will find

application in cases where only moderate degree of conversion (say 60% or less)

is required· The catalyst used in a single-stage process comprises a hydrogenation

function 111 combination with a strong cracking function[ I 0, 15]. The

hydogenation function is providcd by sulphided metals such as cobalt,

molybdenum and nickel. An acidic support, usually alumina, attends to the

cracking function. Nitrogen compounds and ammonia produced by hydrogenation

interfere with acidic activity of the catalyst. In the cases of high/full conversion is

required, the reaction temperatures and run lengths of interest in commercial

operation can no longer be adhered to. It becomes necessary to switch to a multi­

stage process, in which the cracking reaction mainly takes place in an added

reactor. With regard to the adverse effect of ammonia and nitrogen compounds on

catalyst activity, two versions of the multi stage hydrocracker have been

developed: the TWO STAGE HYDROCRACKER and SERIES FLOW

HYDROCRACKER[ I 0,25].

7

In the first type, the undesirable compounds are removed from the unconverted

hydrocarbons before the latter are charged to the cracking reactor. This type is

called the TWO STAGE PROCESS. The other variety is often referred to as

SERIES FLOW HYDROCRACKER. This type uses a catalyst with an increased

tolerance towards nitrogen, both as ammonia and in organic form [ I 0,22].

[II] Two stage hydro cracking

Fresh feed is preheated by heat exchange with effluent from the first reactor. It is

combined with part of a not fresh gas/recycle gas mixture and passes through a

first reactor for desulphurisation/denitrogenation step. These reactions, as well as

those of hydrocracking, which occurs to a limited extent in the first reactor, are

exothermic. The catalyst inventory is therefore divided among a number of fixed

beds. Reaction temperatures are controlled by introducing part of the recycle gas

as a quench medium between beds[ I 0, 17]. The ensuing liquid is fractionated to

remove the product made in the first reactor. Unconverted, material, with low

nitrogen content and free of ammonia, is taken as a bottom stream from the

fractionation section.

After, heat exchange with reactor effluent and mixing with heated recycle gas, it

is sent to the second reactor. Here most of the hydrocracking reactions

occur[ 10,26]. Strongly acidic catalyst with a relatively low hydrogenation activity

(metal sulphides, for example, amorphous silica-alumina) are usually applied. As

in the first reactor, the exothermicity of the process is controlled by using recycle

gas as quench medium the catalyst beds. Effluent from the second reactor is

cooled and joins first stage effluent for separation from recycle gas and

fractionation. The part of the second reactor feed that has remai ned unconverted is

recycled to the reactor. Feedstock is thereby totally converted to the product

boiling range.

8

lSI 2nd .-stage- stage-

Feed

Fresh gas

Quench gos

Recycle gas compressor

Two Stage Hydrocracking

courtuy 01 OSHA www.osha·s1(.~v

Products

Recycle

Figure 1.2 Two Stage Hydro Cracking

[Ill) Series now hydro cracking

The principal difference is the elimination of first stage cooling and gaslliquid

separation and the interstage ammonia removal step. The effluent from the first

stage is mixed with more recycle gas and routed direct to the inlet of the second

reactor. In contrast with the amorphous catalyst of the two-stage process, the

second reactor in series now generally has a zeoli tic catalyst, based on crystalline

silica-alumina[ I 0). As in the two stage process, material not converted to the

product boiling range is recycled from the fractionation section.

9

1.3 Material Selection For Hydro Cracker Reactor[5]

1.3.1 Introduction

Selection of materials of construction in chemical and petrochemical process

industries is of considerable importance for economic operations of plants. Any

material used has to satisfy certain basic requirements of physical, mechanical and

corrosion resistance properties under the operating conditions. Once these are

fulfilled, actual selection depends on number of factors which amongst others

include available capital costs, type of equipment, code requirements,

aggressiveness of environment, product purity requirement, cost of replacement

and maintenance, safety aspects. For the selection of material following factors

should be considered[5]

(I) Mechanical and physical properties of material.

(II) Ease of Equipment fabrication.

(III) Corrosion resistance

(IV) Material maintenance

(I) Mechanical and Physical Properties

Equipments should have mechanical stability at the operating pressure and

temperature conditions. The factors which determine these are! 18]:

(a) Strength of material Higher the strength, lower the thickness of

material used.

(b) Ductility / Fracture toughness: Material should be sufficiently ductile

and tough to avoid any sudden failure because of defects or decrease in

IO

ductility due to metallurgical changes. This property is dependcnt on alloy

composition, heat treatment (hardening), metallurgical changes with time

and temperature related degradations graphitization, nitriding,

carburizing, temperature embrittlement), etc.

(c) Creep properties: Above certain temperature creep properties become

dominant. Higher the temperature and operating stresses more creep and

oxidation restraint material are required.

(d) Thermal conductivity: This property is important in case of heat transfer

servIce.

(e) Thermal expansion I contraction : Where temperature changes are

substantial or where two materials have substantially different coefficient

of expansion, this aspect is to be taken into consideration in the design.

(II) Ease of Equipment Fabrication [65]

In selecting any material of construction, the problems connected with fabrication

and maintenance of equipment are also to be given due consideration. The various

factors are[5,7]:

(a) Conformation to colic requirements or code acceptable material.

(b) Adequate weldability of material.

(c) Expertise available for shop and field welding.

(d) Post weld heat treatment requirements.

(e) Expertise available in plant or with local contractors for maintenance

repairs.

II

".;

(III) Corrosion Resistance

Once mechanical aspects are taken care of, the life of a vessel is governed

primarily by corrosion behavior of the material in the environment to which it is

exposed[5, 15]. Selection of material for resistance to corrosion needs following

considerations.

(a) Operating conditions: Corrosion rate at operating temperature, pressure

and types and concentrations of corrosi ve constituents.

(b) Modification of environment: Whether corrosion can be minimized by

incorporating suitable cOlTosion control methods in the system, e.g.

ncutralization, inhibitor addition, pH control, etc.

(c) Type of corrosive constituents:

(a) In presence of aqueous phase-acid, alkali, salts, H2S, S02.

(b) In absence of aqueous phase H2S, fuel ash, naphthenic acid,

etc.

(d) Type of attack: Uniform pitting, stress corrosion cracking, de-alloying,

hydrogen damage, etc.

(c) Product purity: Allowable metallic impurities in product.

(IV) Material Maintenance

Maintenance plays a vital role to get optimum performance from any vessel. With

best of material selection vessel life will be affected if maintenance and operation

are inadequate. On the other hand, with good maintenance and operation practice

an economic life can be obtained even if less corrosion resistance material is used.

Maintenance involves multi-disciplinary approach and a good maintenance

practice requires the following important inputs[5]:

i) Mechanical inspection/condition monitoring.

ii) Preventive and predictive maintenance.

12

iii) Regular turn-around.

iv) Input of vessel experience.

v) Input of expertise in various related fields.

vi) Keeping abreast with latest developments.

vii) Failure and success analysis.

The older vessels were designed considering maintenance at periodically. This

approach has changed and the present trend is to increase period of continuous

determined performance with in between maintenance period of 2 to 3 years. This

approach requires that no breakdown will occur during this period due to material

failure. The emphasis has therefore shifted to specifying better material, better

inspection and maintenance practices, close control on operation and involved

corrosion control practices.

1.3.2 I>c\'cJopment Stages of Reactor l\laterial [3)

Hydrogen reactors are used for several kinds of processes in refineries and

petrochemical plants, such as desulphurisation of hydrocarbons and cracking of

heavier hydrocarbon fractions into lighter molecules. These processes are carried

out at high temperatures and pressures in the presence of a catalyst.

For morc than last 30 years, conventional low-alloy chromium-molybdenum

2.25Cr-1 Mo steel has been extensively used for the reactor vessels; to a lesser

extent 3Cr-1 Mo steel has been also applied. The reactors generally have been

operated at temperatures lower than 454°C with hydrogen partial pressure above

10Mpa growing demands for higher service temperatures/pressures and increased

reactor sizes resulted in larger and heavier reactors with a unit weight up to 1500

metric tons. Many problems with transportation and construction of such reactors

were encountered. Moreover, due to the process temperature increase dose to the

13

position of 2.25Cr-1 Mo curve according to API 941 (Nelson diagram)[ 13], high

temperature hydrogen attack was possible to occur during the reactors' service.

In order to solve these problems, new generation of vanadium modified Cr-Mo

steels was developed. The steels are standardized according to ASME Boiler and

Pressure Vesscls Code, Section VIII Division 2 :

• 2.2SCr-1 Mo-0.25V steel. approved as Code Case 2098-1 in 1991.

allowing several grades with additions of niobium, calcium. titanium and

boron.

• 3Cr-IMo-0.2SV-Ti-B. approved as Code Case 1961 in 1992,

• 3Cr-1 Mo-0.2SV -Nb-Ca, accepted as Code Case 2153 in 1993.

First two reactors made of 3Cr-1 Mo-0.2SV-Ti-B steel by Japan Steel Work were

completed in 1990 [17). and the reactors made of 3Cr-1 Mo-0.2SV -Nb-Ca were

manufactured in 1994 at Kobe Steel, Tagasako Works in Japan [4). The first

2.2SCr-1 Mo-0.2SV reactor was fabricated by Nuovo Pignone in Italy in 1995. By

the end of 2001 year. 98 vanadium-modified vessels had heen fabricated allover

the world.

The evolution of modern hydroprocessing reactors can be demonstrated through

the following generations.[ 15.25)

(I) First Generation - Pre World War II To ]\lid 1960s.

Forged monobled and multilayer reactor construction for hydrogenation

plants using 2.25'70 to 3.8'70 chrome moly alloys. Generally. little regard was

given to impact toughness.

14

(II) Second Generation-Mid 1960s to 1970s.

The birth of modern hydro processing reactors manufactured from heavy

wall 2.25% chrome moly alloys with 54 joule impact toughness (TT54) at - 10nC

wi th no requirements for temper embrittlement control [15).

(III) Third Generation- 1970s to 1980s.

Reactors with temper embrittlement control to limit the embrittlement

factor (1) to and with increased (TT54) toughness to below-ISnq 17] . In addition,

this generation included consideration of the long term susceptibility to temper

embrittlcment measured by the transition temperature curve shift due to

embrittlement. In this generation, precautions against weld overlay disbonding

were introduced.

(IV) Forth Generation-1980s to 1990s.

Temper embrittlement control reduced by limiting the J factor to 100. The

long term susceptibility to temper embrittlement also further reduced. This

generation saw the (TT54) toughness temperature decreases to below _32 0 C.

(V) Fifth Generation - 1990s.

Ncw grades of vanadium modified 2.25% to 3% chrome steels were

introduced for service with higher strength levels, increased hydrogen attack

resistance and TT54 toughness reduced to below _40 0 C.

1.3.3 Characteristics of Cr-Mo-V SteeIs[12]

AS ME VIII-2 Code Cases requirements for chemical composition of

vanadium modified steels are given in Table 1.2. Advanced steel making

processes, such as a process based on pig iron production in the blast furnace by

steel refining in the BOF converter, ladle metallurgy and vacuum treatment are

15

necessary. They provide an extraordinary low content of tramp elements, such as

tin, arsenic, antimony and low level of phosphorus, sulphur and oxygen.

The three modified alloys utilize vanadium addition to enhance tensile

strength at elevated temperature and creep rupture strength, and to improve

resistance to in-service degradation phenomena, such as temper embrittlement,

high temperature hydrogen attack (HTHA) and hydrogen embrittlement[22]. The

effect of vanadium content on the mechanical properties of 3Cr-1 Mo steel is

shown in Fig. 1.3 . The elevated temperature tensile strength and creep rupture of

3Cr-IMo-0.2SV-Nb-Ca are improved by the addition of Niobium. By the addition

of calcium into Cr-Mo steels, calcium sulphides form instead of manganese

sulphides. In opposite to manganese sulphides, stable calcium sulphides do not

dissolve during welding that results in a decreased susceptibility of weldments to

stress relief cracking caused by grain boundary segregation of sulphur.

Table 1.2 Chemical requirements for V modified Cr-Mo steel[15]

Composition. 2.25Cr·l MO.().25V 3Cr-l Mo·0.25V· Ti·B 3Cr·1Mo·0.25V·Nb-Ca wL% Code Ca se 2098-1 Code Case 1961 Code Case 2151

C 0.10-0.15 0.10-0.15 0.10 - 0.15

Mn 0.30 - 0.60 0.30 - 060 0.30 - 0.60

P max. 0.015 max. 0.015 max.0015

S max. 0.010 max. 0.010 max. 0010

Si max. 0.1 max. 0.1 max. 0.1 Cr 2.00 - 2.50 2.75 - 3.25 2.75 - 3.25 1,10 0.90-1.10 0.90 -1.10 0.90 - 1.10 Cu max. 0.25 max. 0.25 max. 0.25 Ni max. 0.25 max. 0.25 max. 0.25 V 0.25-0.35 0.20 - 0.30 0.20- 0.30 Nb max. 0.07 . 0.015 - 0.070 Ca max. 0.015 . 0.0005 - 0.0150 Ti max. 0.030 0015 - 0.035 . B max. 0.0020 0001 - 0.003 .

16

--.. --'- ''-'D ~~ ____ -L ____ ~ ____ ~ ___ -=-~L-==-==~~i

4(,'(7.-.\ _~~~ 3(10···· ~

.~ , • .&

o 0.. 0 .. 2 0,3 ,I},,, (LS

.... anadium [wt %]

Fig. 1.3 Effect of Vanadium content on mechanical properties of 3Cr-IMo

steel [12]

The intentional addition of boron in 3Cr-IMo-O.25V-Ti-B steel increases the

hardenability and assures uniform distribution of mechanical properties

throughout most heavy cross sections. The addition of titanium helps to maximize

the effect of boron[34].

The modified steels are supplied in quenched and tempered condition. Tempering

is carried out at 690-710°C in order to reduce the strength level and to improve

impact properties of the quenched steels. The tempering process is performed

after the final post weld heat treatment cycle of the reactor. The steels exhibit

some decrease in low temperature toughness compared to the conventional steels:

54 J transition temperature (54 J TT) is -29°C and -40°C, respectively.

Design properties of the conventional Cr-Mo steels and vanadium modified

Cr-Mo steels are summarized in Table 1.3. Higher room temperature strength

properties of the modified steels are presented. Increased mechanical properties

17

allow for higher design stresses, leading to a decreased wall thickness and a

reduced weight of the reactors[ 4].

According to API 941, 0.25 wt.% of vanadium in 2.25Cr-IMo steel protects the

material from HTHA (High Temperature Hydrogen Attack) under hydrogen

partial pressure 13.79 MPa up to 482°C, compared to 454°C for the conventional

2.25Cr- I Mo alloy[80]. However, despite the vanadium addition, the maximum

design temperature permitted by ASME VIII-2 for modified 3Cr- I Mo steels is

454°C at the present time. These steels cannot meet the Division 2 creep rupture

requirements at 482°C. It is indicated in Table 1.3 that application of the

modified steels results in lower unit weight of the reactors at a comparable cost.

Due to the more wall thickness, the high hardenability and the severe servIce

conditions of the reactors made of vanadium modified Cr-Mo steels, welding

needs precaution and careful processing. Welds are always post weld heat treated

before service. Generally, 54J TT of weld metal and base metal are 20°C to 30°C

higher than those of the conventional 2.25Cr- I Mo steel, but the transition

temperature of heat affected zone is sufficiently low enough compared with those

of base metal and weld metal.

1.3.4 ASME Code Requirement for Hydrocracker.

Since hydro processing reactors generally operate in the temperature range of

4000 C to 4540 C with hydrogen partial pressures above 10 MPa, API 94 I has

dictated that as a minimum requirement, the material for the construction of these

reactors be 2.25Cr-IMo steel[17]. As indicated earlier, 2.25Cr- IMo alloys have

been extensively used for the construction of hydro processor reactors in the

present time. 3 Cr- I Mo material has also been used to a lesser extent where

process conditions dictated a requirement for the higher alloy content in the steel.

18

However, the need for higher stress intensity values resulting in reduced wall

thickness reactors has been the driving force for the development of alternate

higher strength materials and this development gained momentum in the early

1980s.[II]

As a result, the followi ng conventional and newer alloy steels can be used for

hydro processor reactors today.

(I) Conventional 2.2SCr-J Mo Alloys

This material has been the alloy material of choice for almost all of the hydro

processing reactors that have been constructed during the past 30 years. The

property requirement and values are given in Table 1.3.

(II) 2.2SCr-lMo Alloy (ASME Code Case 1960-3)

This material is basically the same material as conventional 2.25Cr- I Mo material

except that the final tempering ( Final post weld heat treatment) temperature is

limited to assure a higher tensile strength. This restriction results in higher

permissible design stress intensity values (9% over conventional 2.25 Cr- I Mo

material)[ I 3] . The limitations of this material include a potential for higher

hydrogen embrittlement and hydrogen attack susceptibility as well as a maximum

design temperature limitations of 4540 C. Because of these limitations, this alloy

has found limited acceptance and only a relatively small number of reactors have

been built to date utilizing this code casco

19

(III) 2.2SCr-lMo -0.2SV Alloy ( ASME Code Case 2098-2)[3,4]

Because of vanadium addition and increased tensile strength, this material

achieves the highest permissible design stress intensity values of all these

materials under discussion ( 12% increases at 454°C and 39% increases at 482"C

over conventional 2.25Cr-IMo material). Vanadium addition generally increases

the tensile strength, creep mpture strength and hydrogen attack resistance of these

alloys[ II]. This material has proven to have an excellent resistance to hydrogen

attack through extensive testing during its development. This resistance to

hydrogen attack can be attributed to finely dispersed stable vanadium carbides.

The addition of vanadium enhances the creep mpture life of 2.25 Cr-I Mo alloy to

a greater degree than that for 3Cr- and Cr I Mo alloys.

The API 941 committee recently voted to elevate this material to the same level as

3 Chrome I Moly alloy on the Nelson Curve. Because of its better creep mpture

properties and hydrogen attack resistance, this material is the only vanadium

enhanced material permitted by the ASME Sec-VlIl, Div. 2 code which can be

used with design temperature up to 482"C with hydrogen partial pressure 11.7

MPa, that are typical in hydro processing reactors. ASME Code Case 2098-1

permits this material use up to 4820 C.

The addition of vanadium was originally thought to present disadvantages in

terms of hardenability and weldability and a higher susceptibility to reheat or

stress relief cracking[ 15]. These effects have not been found to be significant.

Recent studies have indicated that low impurity vanadium enhanced 2.25Cr-1 Mo

steel has similar susceptibility to stress relief cracking as the conventional steel.

Other inherent disadvantages of this material in the past included limited

avaibility of welding consumables and the suggested need for intermediate stress

relief at 600° C in lieu of a dehydrogenation treatment usually conducted at

20

315"C[8]. In spite of extensive testing programs that have taken place over a

number of years in the development of this alloy, the first commercial reactor is

only now being constructed by Nuovo Pignone in Italy. A number of weld

manufactures have now also developed welding consumables which give good

metal strength, toughness and creep mpture properties.

(IV) Conventional3Cr-1 Mo alloy:

This material has been used for the construction of hydro processor pressure

equipment on a limited basis where process requirements indicated a need for a

higher chromium content. The disadvantages of this material in comparison of

convention 2.25Cr-1 Mo material include internal stress rupture properties above

426"C and, therefore, lower permissible design stress intensity values (13'70 lower

at 454 "c and higher material costs.)[3]. Further more, the ASME section VJIJ

Division 2 Code limits the maximum design temperature of vessels constructed

from this material to 454 "c. Because of the limited use of this material, welding

consumables are generally not readily available.

(V) 3 Cr- IMo-O.2SV-Ti-B (Formerly C.C. 1961) and 3Cr-1Mo-V-Cb-Ca

alloys (Formerly C.C.2ISI)

These newer alloys, are similar in opportunities with yield of 414 MPa and

minimum and maximum tensile strengths between 586 MPa and 758 MPa. Both

of the alloys are susceptible identically in chemical composition except that the

first alloy contains intentional addition of limited amounts of titanium and boron

and the second alloy contains limited amounts of columbium and calcium.

It is to be noted that both alloys utilize vanadium addition (0.5% V nominal) to

enhance creep rupture strength and hydrogen attack resistance. However despite

21

the vanadium addition, the maximum design temperature permitted by the ASME

Section VIll Division 2 Code at the present time is 454" C because experimental

data suggests that these alloys cannot meet the Division 2 creep rupture

requirements at 482"C. At 454"C, these material have approximately 9 '70 higher

allowable stress intensity values than the conventional 2.25Cr-IMo material.

Intentional addition of boron in the c.c. 1961, material Increases the

hardenability of the material and assures uniform distribution of mechanical

properties through out most heavy cross sections. The addition of titanium helps

to maximize the effect of boron ( Kimure et al 1993)[ II]. The C.C.2151 alloy

utilizes columbium for enhancing tensile strength by precipitation of columbium

carbides (CbC) and minimizes reheat cracking by decreasing intergranular micro

segregation of sulphur (Kobe steel, 1993)[30].

The C.C. 1961 alloy, now patented in the U.S., was developed in the early '80s

and was adopted an ASME code material in 1992. The first commercial reactor

was constructed in 1990 hy Japan Steel Works (J.s.W) to date twenty two

commercial reactors have been built by J.S.W and five more are presently under

construction. The same welding consumables originally developed by the weld

wire decision of Kobe stcel have been used for both materials.

Table 1.3 indicates the relative tensile strength and the AS ME Section VIll

Division 2 design intensity stress values of these materials along with other

relevant data. Each of the alloys characteristics related to the design and

construction of hydro processing reactors are available in either plate or forging

product forms[ 12]. Also included in Table 1.3 is relative cost/weight figures

based on a typical nominal 1,000 ton hydro processor reactor fabricated from each

of the alloy discussed. At a design temperature of 454"C, the enhanced 2.25Cr-

22

I Mo alloys provides a marginally lower total cost over the conventional 2.2SCr­

I Mo steel and the vanadium mouified steels[ I 0].

However, the limitations of this material have precluded its general acceptance.

The vanadium-modified steels provide,

• The lightest reactors, which can be a significant benefit III foundation

design, transportation and erection.

• At a design temperature of 482"C, 2.2SCr-1 Mo-0.2SV steel offers a very

substantial advanced from reduced weight and cost considerations

• Higher tensile strength at room and design temperature thereby allowing

higher allowable stress, which in turns results in to thinner wall thickness.

• Bctter elevated temperature properties.

• Bctter hydrogen embrittlement and hydrogen attack resistance.

• Better temper embrittlemcnt resistance.

• Good weldability.

It is the only material, which can be, used hydrogen partial pressures above I 1.7

MPa.[ I 0,4]

23

Table 1.3 Comparison of Hydro processing reactor materials [4]

Alloy Types Conventional Enhanced 2.25Cr·tMo· 3Cr·tMo· 3Cr·1Mo·V· 2.25Cr·1Mo 2.25Cr·1Mo O.2SV O.25V·Ti·B Cb·Ca

Old ASME Code IIA 1 '~":,')-3 200S-2 196-1 2'lt,! Case

ASME Material SA336 F22 SA5-11 Ll2L'I) SA:r2>t, F22V ~A~i~~(, f 3V '::./\2/36 F 3'.1 Specifications

SA387 Gr22 SA5-12 [~-B CI2 SAS 12 Tr-D SA5~2 [p-C S/.S12 Tp-C CI2 Clb CI·lo Clio

ASME Sec VIII Dlv.2 Max Allowable Temp 182 454 ·IS2 ~,,4 I~,·I

(Deg.C) Room Temp. UTS

517 -689 585-760 5B5-760 58,,-760 5035-760 (Mpa) Room Temp. YS 31u I',lin 37'd f'.lin ~ 111',lin 414 Min 411 1·.lin

(Mpa) Design Stress 159@'154C 161 @ 454C H,9 r@ 454"C 16-1 @'I5cIC E·I .g:. ·1',4·C

Intensity as per ASME Sec VIII Dlv.2 117 @182C l'IA(i'j; ,182(: -\,)3 I.g,' -IB2"C I·IA ,:iji 482(: tlA I,IJ' 482-C

(Mpa)

Nelson Curve for '151"(:- 45·1'(:- 51'XC- :E,4 C- 4'.,1 (;-

Max Temp In H2 I pp",>= 11l1dPa ppe" = l1.7I'.'1Pb pp..>=12.4f.1Pa I pp~>=12.".IP8 pp,,,>=1241',lPa 482 C- 432·C· Service

r'D .. >=13.8MPa , PPei"=13.BI·.1Pa Typical Vessel :-r2.8mlll@

307ml11 (<1' ,",rc 2~'t'mm IW :.;u7mm :g:. -'tl,"mrn -G' Thickness as per ·I' .. I"C ·154C 4S'IC -I~, I'C

ASME Sec VIII. Dlv-2 4-t 2 III III I~IT' 3"!I:mm@ for a 1000MT IIA@ 482'C NA@,lfl2C f".Jl'l ;J" IUC

Reactor ,182'C 482C

All the alloys described are supplied in the quenched and tempered condition with

vacuum degassing to achieve high impact properties. Quenching is usually carried

out in a water filled quench tank fitted with high capacity re-circulating pumps to

agitate the water for maximum possible quenching effects. Tempering is normally

calTied out at 690"C - 704°C for the conventional alloys and 704"C - 7IS"C for

vanadium-modified alloys in order to soften and reduce the strength level of the

quenched steel and to improve its impact properties. The tempering process is

carried out during the final post weld heat treatment cycle of the reactor. The steel

is usually ordered on the basis of allowing for two additional post weld heat

treatment cycles for future weld repairs or modifications in the field if needed[4].

All the above discussion leads to one important point that selection of hydro

cracker reactor material needs lots of economical and technical consideration.

24

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2S

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26

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28

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29

-----------------------------------------------------------------

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30