U.S. patent application number 16/386299 was filed with the patent office on 2020-06-04 for low-alloy heat-resistant steel having high reheat-cracking resistance.
The applicant listed for this patent is Wuhan University. Invention is credited to Shiyu JING, Jian LI, Yong LI, Xue WANG, Caiying XIA, Huachun YANG.
Application Number | 20200173001 16/386299 |
Document ID | / |
Family ID | 65186637 |
Filed Date | 2020-06-04 |
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United States Patent
Application |
20200173001 |
Kind Code |
A1 |
WANG; Xue ; et al. |
June 4, 2020 |
LOW-ALLOY HEAT-RESISTANT STEEL HAVING HIGH REHEAT-CRACKING
RESISTANCE
Abstract
Steel, including the following element ratios (in mass percent
amounts): carbon (C): 0.04-0.11%; silicon (Si): 0.50% or less;
manganese (Mn): 0.10-0.60%; phosphorus (P): 0.03% or less; sulphur
(S): 0.01% or less; nicide (Ni): 0.40% or less; chromium (Cr):
1.90-2.60%; vanadium (V): 0.20-0.30%; niobium (Nb): 0.02-0.08%;
molybdenum (Mo): 0.05-0.30%; tungsten (W): 1.45-1.75%; titanium
(Ti): 0.01-0.06%; boron (B): 0.001-0.012%; aluminum (Al): 0.03% or
less; nitrogen (N): 0.01% or less; the balance being iron (Fe) and
impurities. The contents of C and B satisfy the following
inequality: (% B) -1.2.times.(% C).sup.2+0.30.times.(% C)-0.01.
Inventors: |
WANG; Xue; (Wuhan, CN)
; LI; Yong; (Wuhan, CN) ; LI; Jian; (Wuhan,
CN) ; YANG; Huachun; (Wuhan, CN) ; JING;
Shiyu; (Wuhan, CN) ; XIA; Caiying; (Wuhan,
CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wuhan University |
Wuhan |
|
CN |
|
|
Family ID: |
65186637 |
Appl. No.: |
16/386299 |
Filed: |
April 17, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/54 20130101;
C22C 38/04 20130101; C22C 38/48 20130101; C22C 38/001 20130101;
C22C 38/46 20130101; C22C 38/50 20130101; C22C 38/44 20130101; C22C
38/002 20130101; C22C 38/02 20130101; C22C 38/06 20130101 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2018 |
CN |
201811462079.4 |
Claims
1. Steel, comprising the following element ratios in mass percent
amounts: carbon (C): 0.04-0.11%; silicon (Si): 0.50% or less;
manganese (Mn): 0.10-0.60%; phosphorus (P): 0.03% or less; sulphur
(5): 0.01% or less; nickle (Ni): 0.40% or less; chromium (Cr):
1.90-2.60%; vanadium (V): 0.20-0.30%; niobium (Nb): 0.02-0.08%;
molybdenum (Mo): 0.05-0.30%; tungsten (W): 1.45-1.75%; titanium
(Ti): 0.01-0.06%; boron (B): 0.001-0.012%; aluminum (Al): 0.03% or
less; and nitrogen (N): 0.01% or less, the balance being iron (Fe)
and impurities; wherein the contents of C and B, in mass percent
amounts, satisfy the following inequality: (% B)>-1.2.times.(%
C).sup.2+0.30.times.(% C)-0.01.
2. The steel of claim 1, content of C in the steel is
0.04-0.08%.
3. The steel of claim 1, wherein a content of B in the steel is
0.004-0.01%.
4. The steel of claim 1. wherein a content ofB in the steel is
0.006-0.01%.
5. The steel of claim 1. wherein the contents of C and B satisfy
the following inequality: (% B)>-1.4.times.(%
C).sup.2+0.35.times.(% C)-0.0115.
6. The steel of claim 2, wherein the content of B in the steel is
0.004-0.01%.
7. The steel of claim 2, wherein the content of B in the steel is
0.004-0.008%.
8. The steel of claim 1, wherein a content of C in the steel is
0.04-0.08%, and the contents of C and B satisfy the following
inequality: (% B)>-1.4(% C).sup.2+0.35.times.(% C)-0.0115.
9. The steel of claim 8, wherein the content of B in the steel is
0.004-0.01%.
10. The steel of claim 8, wherein the content of B in the steel is
0.006-0.01%.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Pursuant to 35 U.S.C. .sctn. 119 and the Paris Convention
Treaty, this application claims foreign priority to Chinese Patent
Application No. 201811462079.4 filed Nov. 30, 2018, the contents of
which, including any intervening amendments thereto, are
incorporated herein by reference. Inquiries from the public to
applicants or assignees concerning this document or the related
applications should be directed to: Matthias Scholl P.C., Attn.:
Dr. Matthias Scholl Esq., 245 First Street, 18th Floor, Cambridge,
Mass. 02142.
BACKGROUND
[0002] The disclosure relates to a heat-resistant steel, which has
excellent resistance to reheat cracks, and is insensitive to
intergranular cracks.
[0003] The reheat cracking, also referred to as a post-weld heat
treatment cracking or a stress relief cracking, refers to
intergranular cracking generated in a coarse-grained heat affected
zone (CGHAZ) of a welded joint of a metal material due to stress
release during a post-weld heat treatment or high-temperature
service.
[0004] Conventional low-alloy (i.e., having less than 5 wt. % of
alto-resistant steels, such as T23, T24, Cr--Mo, and Cr--Mo--V,
tend to reheat cracking.
SUMMARY
[0005] The disclosure provides steel, which has high reheat
cracking resistance in a coarse-grained heat affected zone while
maintaining excellent high-temperature creep strength, and is
insensitive to reheat cracking during a post-weld heat treatment or
service at temperatures of 500 to 750.degree. C.
[0006] Disclosed is steel, comprising the following element ratios,
stated in mass percent amounts: carbon (C): 0.04-0.11%; silicon
(Si): 0.50% or less; manganese (Mn): 0.10-0.60%; phosphorus (P):
0.03% or less; sulphur (S): 0.01% or less; nickle (Ni): 0.40% or
less; chromium (Cr): 1.90-2.60%; vanadium (V): 0.20-0.30%; niobium
(Nb): 0.02-0.08%; molybdenum (Mo): 0.05-0.30%; tungsten (W):
1.45-1.75%; titanium (Ti): 0.01-0.06%; boron (B): 0.001-0.012%;
aluminum (Al): 0.03% or less; and nitrogen (N): 0.01% or less, the
balance being iron (Fe) and impurities; where the contents of
carbon (C) and boron (B) satisfy the following inequality:
(% B)>-1.2.times.(% C).sup.2+0.30.times.(% C)-0.01 (1).
[0007] Unless otherwise stated, the percent (%) of chemical
compositions in the disclosure refers to a mass percent.
[0008] The content of C in the steel is 0.04-0.08%, and the content
of B in the steel can be 0.004-0.01%.
[0009] The content of C in the steel can be 0.04-0.08%, and the
content of B in the steel can be 0.004-0.008%.
[0010] The content of B in the steel can be 0.004-0.012%.
[0011] The content of B in the steel can be 0.006-0.010%.
[0012] The contents of C and B in the steel satisfy the following
inequality:
(% B)>-1.4.times.(% C).sup.2+0.35.times.(% C)-0.0115 (2).
[0013] Also provided is a heat-resistant steel comprising the
following element ratios in mass percent amounts: C: 0.04-0.08%,
Si: 0.50% or less, Mn: 0.10-0.60%, P: 0.03% or less, 5: 0.01% or
less, Ni: 0.40% or less, Cr: 1.90-2.60%, V: 0.20-0.30%, Nb:
0.02-0.08%, Mo: 0.05-0.30%, W: 1.45-1.75%, Ti: 0.01-0.06%, B:
0.001-0.012%, Al: 0.03% or less, and N: 0.01% or less, the balance
being Fe and impurities, where the contents of C and 13 satisfy the
following inequality:
(% B)>-1.4.times.(% C).sup.2+0.35.times.(% C)-0.0115 (2)
[0014] The content of B in the steel can be 0.004-0.01%.
[0015] The content of B in the steel can be 0.006-0.01%.
[0016] The function of each element in the steel of the disclosure
and the effects of the range of each element will be described
below
[0017] C: 0.04-0.11%
[0018] C is formed into carbide in the steel, which is good for
high-temperature strength. Further, C helps to improve
hardenability and avoid forming a ferrite. Therefore, the content
of C is at least 0.04%. However, the excessive C increases the
hardness of a heat affected zone of welded joint, and increases
sensitivity to cold cracks, and particularly contributes to reheat
cracking. In addition, steel with a high content of C may become
brittle after being used for a long time at high temperature.
Therefore, an upper limit of the content of C is 0.11%, and
preferably 0.04-0.08%.
[0019] Si: 0.50% or less
[0020] Si serves as a deoxidizing element in steel manufacture. Si
is also effective for improving oxidation resistance and
high-temperature corrosion resistance of the steel. However, the
excessive content of Si may result in reduction of creep plasticity
and toughness during long-term use at high temperature. Therefore,
the upper limit of the content of Si is 0.50%, and a lower limit is
an unavoidable impurity content level. To ensure deoxidization
effect, the content of Si preferably is 0.10-0.30%.
[0021] Mn: 0.10-0.60%
[0022] Like Si, Mn is added as a deoxidizing agent. However,
excessive addition of Mn may result in creep embrittlement and the
reduction of toughness. Therefore, the maximum content of Mn is
0.60%. To ensure the deoxidization effect, the content of Mn
preferably is 0.20-0.50%.
[0023] P: 0.03% or less
[0024] P exists as an unavoidable impurity in the steel. The high
content of P may easily result in reheat cracking. Therefore, the
maximum content of P is 0.03%. The content of P preferably is as
low as possible, and therefore, no lower limit is set for the
content. However, excessive reduction of the content of P may
increase manufacturing cost, and thus the content of P preferably
is 0.001-0.01%.
[0025] S: 0.01% or less
[0026] Like P, S also exists as an unavoidable impurity. S may be
easily segregated in the CGHAZ, thereby resulting in the generation
of reheat cracking. Therefore, the content of S is limited to 0.01%
or less. The content of S preferably is as low as possible, and
therefore no lower limit is set for the content. However, like P,
excessive reduction of the content of S may increase the
manufacturing cost, and thus the content preferably is
0.002-0.006%.
[0027] Ni: 0.4%.COPYRGT. or less
[0028] Ni is an austenite forming element. Ni inhibits the
formation of a 8 `ferrite phase and ensures the stability of a
ferrite structure. Excessive addition of Ni may reduce the
plasticity during high-temperature use. Therefore, the content of
Ni is limited to 0.4% or less.
[0029] Cr: 1.90-2.60%
[0030] Cr is an indispensable element for ensuring high-temperature
oxidization resistance, high-temperature corrosion resistance and
high-temperature strength. However, excessive addition of Cr may
coarsen the carbide, thereby finally resulting in the reduction of
high-temperature strength and toughness. Therefore, the content of
Cr is limited to 1.90-2.60%.
[0031] `V: 0.20-0.30%
[0032] V is formed into fine carbides or carbonitrides in the
steel, and helps to increase creep strength. However, excessive
addition of V may result in increased growth speed of the carbides,
premature aggregation and coarsening, and as a result premature
disappearance of dispersion strengthening and toughness reduction.
In addition, excessive addition of V may increase precipitation
density of the carbonitrides inside grain during a post-weld heat
treatment and increase the reheat cracking sensitivity. Therefore,
the content of V is limited to 0.20-0.30%.
[0033] Nb: 0.02-0.08%
[0034] Nb is formed into fine and stable carbides or carbonitrides
in the steel, and helps to increase creep strength. Therefore, it
is necessary to add at least 0.02% of Nb. However, excessive
addition of Nb may result in the increased growth speed of the
carbides, premature aggregation and coarsening, and consequently
premature disappearance of dispersion strengthening and toughness
reduction. Therefore, the content of Nb is limited to
0.02-0.08%.
[0035] Mo: 0.05-0.35%
[0036] Mo increases the solid solution strength of steel matrix,
and is precipitated in the form of carbides to increase the creep
strength. Further, Mo has strong affinity for P, and may reduce an
amount of P segregated at a grain boundary, thereby helping to
reduce the reheat cracking sensitivity. Therefore, it is necessary
to control the content of Mo to 0.05% or more. However, excessive
addition of Mo may reduce the toughness of the steel after
long-term exposure and thus, the upper limit of Mo is 0.35%.
[0037] W: 1.45-1.75%
[0038] Like Mo, W increases the solid solution strength of the
steel matrix, and is formed into carbides to increase the creep
strength. To obtain these effects, the content of W is at least
1.45%, but excessive addition may result in the generation of
coarse intermetallic compounds during service and further reduce
toughness. Therefore, the content is limited to less than
1.75%.
[0039] Ti: 0.01-0.06%
[0040] The addition of Ti may bind N to prevent a combination of N
and B. In this way, the hardena.bility is improved to prevent the
reduction of tensile strength due to the generation of the ferrite,
and also grain boundary strength and plasticity at a high
temperature are increased. However, the excessive content of Ti may
reduce the strength and toughness, and also increase the reheat
cracking sensitivity. Therefore, the content of Ti is in a range of
0.01-0.06%.
[0041] B:0:001-0.012%
[0042] B can increase the creep strength and the creep fracture
plasticity. Further, during the post-weld heat treatment, B can
increase the grain boundary plasticity through grain boundary
segregation in the CGHAZ and also can inhibit the precipitation,
growth and coarsening of the carbides, prevent the grain boundary
from weakening, and reduce the reheat cracking sensitivity. To be
effective, the contents of B and C must satisfy (% B)-1.2.times.(%
C).sup.2+0.30.times.(% C)-0.01. Preferably, when the contents of B
and C satisfy (% B) >-1.4.times.(% C).sup.2+0.35.times.(%
C)-0.0115, better reheat cracking resistance can be achieved. The
excessive content of B may obviously deteriorate hot workability of
the steel and also cause temper embrittlement. Therefore, the
content of the element B appropriately is 0.001-0.012%, preferably
is 0.004-0.010%, and more preferably is 0.006-0.010%.
[0043] Al: 0.03% or less
[0044] Al is included as a deoxidizing agent. However, since the
excessive content may result in the reduction of creep plasticity
and toughness, the content of Al is limited to 0.03% or less.
[0045] N: 0.01% or less
[0046] N is soluble in the matrix and thus is harmful to the
toughness and creep strength. Further, the excessive content of N
may form compounds with B, which is not favorable to exerting the
function of B. Therefore, the content of N is limited to 0.01% or
less.
[0047] Advantages of the steel of the disclosure are summarized as
follows. The purpose of inhibiting reheat cracking of a low-alloy
heat-resistant steel can be achieved by adjusting the contents of
two elements C and B, only. Compared with the demanding welding
process adopted for preventing reheat cracking, the disclosure
solves the problem of reheat cracking sensitivity, and does not
increase production cost in order to achieve a more reliable
result.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 shows a relationship of M.sub.23C.sub.6, an MX type
carbide, and a carbon content in a 2.25Cr-1.6WVNbNB steel.
[0049] FIG. 2 illustrates a microscopic structure of steel.
[0050] FIG. 3 illustrates a sample shape in an implant test.
[0051] FIG. 4 illustrates a simulated thermal cycle curve and a
sample shape and size.
[0052] FIG. 5 is a schematic diagram illustrating a relationship
between the contents of B and C of a heat-resistant steel according
to one embodiment of the disclosure.
[0053] In the drawings, the following reference numbers are used:
1. Bottom plate; 2. Implant; and 3. Loading direction.
DETAILED DESCRIPTION
[0054] To further illustrate, embodiments detailing a
heat-resistant steel are described below. It should be noted that
the following embodiments are intended to describe and not to limit
the disclosure.
[0055] The disclosure provides a low-alloy heat-resistant steel
with reheat cracking resistance, which greatly improves a reheat
cracking resistance while maintaining excellent high-temperature
creep strength and low-welding cold cracking sensitivity of the new
low-alloy heat-resistant steel. The steel can be safely and
reliably used in an ultra (super) critical thermal power plant.
[0056] Chemical composition of steel influences its sensitivity to
reheat cracking. In particular, alloy elements such as C and W, Mo,
V, Nb and Ti may all increase the tendency to undergo reheat
cracking. However, W and Mo are soluble in the steel matrix and
provide solid solution strengthening, the V, Nb, and Ti are formed
into fine dispersed carbonitrides to provide precipitation
strengthening, and thus they are key elements for increasing a
high-temperature creep strength of a material. To obtain sufficient
high-temperature creep strength, it is necessary to add a certain
amount of these alloy elements, The inventor of the disclosure
studied the reheat cracking generation mechanism of the low-alloy
heat-resistant steel T23 and found that the content of impurity
elements in this steel can be controlled to be in a low range with
the development of steel manufacturing technology and that this is
no longer a leading factor for generating reheat cracking, but that
the precipitation of alloy carbides during a post-weld heat
treatment has a significant impact on the generation of reheat
cracking. The above is different from the past viewpoint that
emphasizes the segregation of impurities elements on the grain
boundary of the conventional low-alloy heat-resistant steel, and
weakens the grain boundary, resulting in the generation of reheat
cracks. Thus, a new basis for developing a new low-alloy
heat-resistant steel with reheat cracking resistance is
provided.
[0057] The following findings are offered with respect to the
reheat cracking mechanism of the CGHAZ in T23 steel.
[0058] (1) Most of the carbides in the CGHAZ are soluble in the
matrix during welding, and M.sub.23C.sub.6 type carbides containing
elements such as Fe, Cr, W and Mo are precipitated along the grain
boundary, rapidly grown, and coarsened to reduce the binding force
between grain boundaries during a post-weld heat treatment.
[0059] (2) A relatively large amount of coarse M.sub.23C.sub.6 type
carbides precipitated on the grain boundary within short time may
result in a depletion of alloy elements in the nearby matrix,
thereby forming a softening zone in which strain is preferentially
aggregated under tensile stress at high temperature.
[0060] (3) The incoherent M.sub.23C.sub.6 carbides may also promote
nucleation of a creep cavity on the grain boundary to further
accelerate the weakening of the grain boundary.
[0061] (4) During a short-time post-weld heat treatment, carbides
precipitated in the grain interior mainly are M.sub.23C.sub.6 and
M.sub.3C.sub.3 having small sizes, and may have a certain
strengthening effect, thereby increasing intragranular
strength.
[0062] (5) The intra-granular strength is high and the grain
boundary is obviously weakened. The grain boundary of the CGHAZ
preferentially deforms to form a cavity under the action of welding
residual stress as well as thermal stress, and the cavities are
aggregated to form micro-cracks and propagate further to cause
inter-granular fracture.
[0063] Carbides formed by elements such as Cr, W, and Mo have a
large impact on reheat cracking of the heat-resistant steels, but
the elements such as V, Nb and Ti have a small impact on reheat
cracking. Therefore, from the perspective of inhibiting the reheat
cracking, the precipitation of M.sub.23C.sub.6 type carbides on the
grain boundary and the grain interior of the CGHAZ are to be mainly
controlled.
[0064] Carbon is an essential element for forming M.sub.23C.sub.6,
the higher carbon content of the steel may cause more carbon atoms
to form carbides in the CGHAZ during a post-weld heat treatment.
Therefore, a precipitation amount of CGHAZ carbides may be limited
by limiting the content of carbon. FIG. 1 illustrates a change of
the contents of M.sub.23C.sub.6 and MX carbides in a steel T23
along with the content of the carbon element, where the contents of
M.sub.23C.sub.6 and MX carbides and carbon are calculated by using
Thermo-calc software (where a total amount of an alloy system is 1
mol, and a result of a metastable state is obtained by inhibiting
the generation of an M.sub.6C phase during calculation). As shown,
the contents of both MX and M.sub.23C.sub.6 phases linearly
increase along with the increase of the content of C, but the
increase of the content of the M.sub.23C.sub.6 phase is greater
than that of the MX phase. When the content of C increases by each
0.02%, the M.sub.23C.sub.6 type carbide in 1 mol of the steel
increases by about 0.003 mol, and the MX phase increases only by
about 0.0002 mol which is less than a tenth of the former. That is,
reducing the content of C can greatly reduce the content of
M.sub.23C.sub.6 type carbide, but has a small impact on the MX type
carbide. The MX phase is a main precipitation strengthening phase
in the grain and is extremely important for maintaining
high-temperature creep strength. The strengthening effect of the MX
phase in the grain interior may still be maintained since the
appropriate control of the carbon content has a small impact on the
MX phase, thereby ensuring the high-temperature creep strength.
Therefore, it is appropriate to reasonably limit the carbon content
in consideration of reducing the content of carbides on the grain
boundary and lessening the weakening of the grain boundary of the
CGHAZ during the post-weld heat treatment.
[0065] Further, the weakening of the grain boundary caused by grain
boundary carbides is related to the size and distribution of the
carbides. The coarse carbides may reduce coherence and cause severe
depletion of alloy elements in the matrix near the grain boundary,
thereby intensifying the weakening of the grain boundary. However,
the reduction of coherence and the depletion of elements caused by
the fine carbides are much smaller. The reheat cracking sensitivity
of the steel T23 is high because the M.sub.23C.sub.6 type carbides
precipitated on the grain boundary of the CGHAZ during a post-weld
heat treatment are extremely easily aggregated and coarsened. The
element B is easily segregated in a vacancy of the grain boundary
in the steel to inhibit the segregation of impurity elements,
reduce activation energy of the grain boundary, purify and
strengthen the grain boundary, and improve the plasticity of the
grain boundary. The element B can also enter into the
M.sub.23C.sub.6 phase to form a more stable M.sub.23(C, B).sub.6
phase to inhibit its coarsening. The addition amount of B is
related to the content of C in the steel. The higher content of C
indicates that more B needs to be added to inhibit the aggregation
and coarsening of the M.sub.23C.sub.6 phase.
[0066] Therefore, the contents of C and. B in the steel are
controlled. This reduces the amount and the size of the
M.sub.23C.sub.6 type carbides precipitated in the grain boundary
during a post-weld heat treatment, improves the plasticity of the
grain boundary, and reduces the difference between grain boundary
strength and intragranular strength, thereby inhibiting the
generation of reheat cracking.
[0067] FIG. 5 obtained by using data in the following several
examples illustrates a relationship between the content of B
existing in the steel and the generation of reheat cracks relative
to the content of C. In FIG. 5, the horizontal axis refers to the
content of C in the steel according to examples of the disclosure
or comparison examples, and the longitudinal axis refers to the
content of B in the steel. A hollow circle (a) refers to steels
with no reheat cracks, and a solid circle (.circle-solid.) refers
to steels with reheat cracks. It is determined from FIG. 5 that the
relationship between the content of C (% C) and the content of B (%
B) satisfies the following inequality:
(% B)-1.2.times.(% C).sup.2+0.30.times.(% C)-0.01 (1),
and thus, the reheat cracks can be prevented.
[0068] Steels comprising chemical compositions (containing a
maximum total amount of 0.04% of impurities such as Sn, As, Sb, Bi
and Pb) shown in Table 1 are prepared. Steel billets are obtained
by electric arc furnace refining plus external refining and vacuum
degassing or smelting by an electroslag remelting process. Bars
with a size being 55 mm.times.55 mm.times.800 mm are manufactured
by hot forging; pipes with different specifications are
manufactured by perforation and hot rolling, outer diameters of the
pipes cover a plurality of sizes from 38.1 mm to 63.5 mm, and wall
thickness covers a plurality of sizes from 4.5 mm to 10 mm. Heat
treatment of normalization and tempering are performed for plates
and pipes. The normalizing temperature was 1060.degree. C., and the
plates and pipes were air-cooled after being incubated for 2 hours.
The tempering temperature is 760.degree. C., and the plates and
pipes are cooled in the furnace after being incubated for 1 hour. A
microstructure of the steel of the disclosure is a full bainite
steel, as shown in FIG. 2.
[0069] The reheat cracking sensitivity was evaluated by performing
a test with a sample. The reheat cracking sensitivity of the steel
is evaluated by three methods in the disclosure, and the methods
comprise a post-weld heat treatment test of welded joints performed
in a waterwall pannel and a header, an implant test and an
isothermal slow strain rate tensile test of simulated CGHAZ.
[0070] A process of performing an actual post-weld heat treatment
test of a joint comprises: a welded joint of tube is released
residual stress by performing heat treatment at 730.+-.10.degree.
C. for 0.5 hours after welding, and performing magnetic powder
inspection and X-ray inspection for the joint to detect whether
cracks are generated on a surface of the joint or inside the
joint.
[0071] An implant sampling process comprises: processing samples of
an implant 2 and a bottom plate 1 as shown in FIG. 3, assembling
the implant 2 into the central hole of the bottom plate 1, and
applying a welding pass on the bottom plate. The implant and the
bottom plate that are welded are placed for 24 hours to eliminate
the impact of cold cracks, then installed on an implant tester,
heated to a testing temperature and incubated for 15 minutes, and
then loaded with a particular initial stress. The reheat cracking
sensitivity may be determined by fracture time (t.sub.f). If
t.sub.f is less than 24 hours, it is considered to be sensitive; if
t.sub.f is greater than 24 hours, it is considered to be
insensitive. The shorter the fracture time is, the larger the
reheat cracking sensitivity is.
[0072] A process of performing the isothermal slow strain rate
tensile test comprises: processing samples as shown in FIG. 4,
using the range of 10 mm in the middle as a gauge length portion,
and completing a welding simulation test of a CGHAZ on a thermal
simulator. A simulated thermal cycle curve of welding is shown in
FIG. 3, and a simulated welding process is as follows: TIG welding
process, preheating at 100.degree. C., and a heat input being 25
kJ/cm, After being cooled, the sample is heated to a test
temperature of 500 to 750.degree. C., incubated for 5 seconds, and
then strained at a constant strain rate of 0.5 mm/min until it is
fractured, and a reduction of area of the fractured sample is
measured. According to the reduction of area (Z) of the fractured
sample, the reheat cracking sensitivity of the material may be
generally determined based on the following criteria: 1) the
material is very sensitive when Z is less than 5%; 2) the material
is sensitive when Z is greater than 5% and less than 10%; 3) the
material is slightly sensitive when Z is greater than 10% and less
than 20%; 4) the material is insensitive when Z is greater than
20%. In the disclosure, it is considered as non-cracked when Z is
greater than 20% at all test temperatures and otherwise it is
considered as cracked.
TABLE-US-00001 TABLE 1 Chemical compositions of steels in examples
(wt. %) Chemical compositions (mass percent; balance: Fe and
impurities Steel C Si Mn P S Ni Cr V Nb Mo Comparison 1# 0.052 0.16
0.35 0.006 0.004 0.055 2.22 0.25 0.038 0.11 examples 2# 0.061 0.23
0.35 0.010 0.003 0.014 2.31 0.26 0.056 0.11 3# 0.068 0.23 0.33
0.008 0.003 0.023 2.19 9.24 0.041 0.11 4# 0.068 0.20 0.37 0.008
0.002 0.055 2.35 0.26 0.029 0.13 5# 0.072 0.25 0.36 0.014 0.001
0.022 2.28 0.25 0.050 0.12 6# 0.074 0.23 0.37 0.010 0.006 0.023
2.29 0.23 0.040 0.10 7# 0.049 0.25 0.44 0.014 0.001 0.020 2.25 0.24
0.050 0.11 8# 0.078 0.24 0.38 0.007 0.002 0.030 2.28 0.25 0.030
0.12 9# 0.083 0.16 0.40 0.007 0.001 0.040 2.04 0.23 0.045 0.13 10#
0.076 0.24 0.36 0.011 0.007 0.028 2.16 0.24 0.030 0.09 11# 0.085
0.17 0.35 0.004 0.003 0.005 2.16 0.25 0.038 0.11 12# 0.092 0.16
0.40 0.007 0.001 0.040 2.04 0.23 0.045 0.13 13# 0.10 0.14 0.46
0.014 0.001 0.040 2.20 0.25 0.050 0.12 Examples 14# 0.050 0.27 0.39
0.005 0.002 0.037 2.30 0.24 0.050 0.08 of the 15# 0.056 0.17 0.35
0.005 0.002 0.10 2.15 0.25 0.038 0.12 disclosure 16# 0.050 0.25
0.40 0.004 0.003 0.024 2.24 0.25 0.044 0.12 17# 0.060 0.18 0.35
0.004 0.003 0.002 2.19 0.25 0.040 0.11 18# 0.067 0.21 0.35 0.006
0.002 0.080 2.15 0.24 0.038 0.12 19# 0.075 0.18 0.36 0.005 0.003
0.002 2.18 0.25 0.038 0.12 20# 0.096 0.23 0.38 0.005 0.004 0.012
2.11 0.24 0.042 0.11 21# 0.054 0.18 0.36 0.005 0.002 0.060 2.18
0.25 0.042 0.12 22# 0.060 0.24 0.18 0.017 0.005 0.020 2.18 0.25
0.040 0.07 23# 0.041 0.16 0.35 0.004 0.003 0.003 2.16 0.24 0.035
0.11 24# 0.043 0.16 0.35 0.004 0.003 0.003 2.20 0.25 0.037 0.11 25#
0.050 0.21 0.38 0.006 0.003 0.050 2.26 0.26 0.055 0.09 26# 0.061
0.25 0.22 0.020 0.005 0.010 2.15 0.25 0.038 0.08 27# 0.070 0.18
0.33 0.005 0.001 0.003 2.17 0.25 0.036 0.10 28# 0.075 0.18 0.38
0.006 0.003 0.002 2.18 0.25 0.038 0.11 29# 0.064 0.16 0.42 0.004
0.001 0.004 2.13 0.24 0.042 0.11 30# 0.080 0.18 0.32 0.004 0.002
0.005 2.15 0.25 0.040 0.11 31# 0.089 0.17 0.35 0.004 0.002 0.003
2.18 0.25 0.038 0.12 32# 0.105 0.25 0.42 0.004 0.003 0.008 2.25
0.25 0.044 0.10 Chemical compositions (mass percent; balance: Fe
and impurities) -1.2 * C.sup.2 + Steel W Ti Al B N 0.3 * C - 0.01
Comparison 1# 1.50 0.002 0.020 0.0012 0.008 0.0024 examples 2# 1.56
0.004 0.018 0.0022 0.006 0.0038 3# 1.51 0.004 0.003 0.0011 0.006
0.0049 4# 1.63 0.002 0.020 0.0026 0.006 0.0049 5# 1.53 0.003 0.001
0.0013 0.004 0.0054 6# 1.56 0.005 0.013 0.0030 0.006 0.0056 7# 1.60
0.013 0.001 0.0010 0.003 0.0018 8# 1.60 0.014 0.030 0.0023 0.004
0.0061 9# 1.35 0.003 0.005 0.0036 0.001 0.0066 10# 1.50 0.004 0.006
0.0022 0.006 0.0059 11# 1.56 0.027 0.008 0.0065 0.003 0.0068 12#
1.35 0.003 0.005 0.0048 0.003 0.0074 13# 1.54 0.016 0.022 0.0070
0.008 0.0080 Examples 14# 1.56 0.038 0.009 0.0025 0.006 0.0020 of
the 15# 1.56 0.029 0.013 0.0035 0.002 0.0030 disclosure 16# 1.55
0.033 0.007 0.0030 0.005 0.0020 17# 1.57 0.030 0.010 0.0045 0.001
0.0037 18# 1.56 0.025 0.013 0.0050 0.002 0.0047 19# 1.58 0.022
0.006 0.0063 0.002 0.0058 20# 1.61 0.025 0.008 0.0081 0.006 0.0077
21# 1.55 0.023 0.011 0.0060 0.004 0.0027 22# 1.50 0.025 0.003
0.0055 0.007 0.0037 23# 1.60 0.021 0.007 0.0010 0.002 0.0003 24#
1.57 0.024 0.004 0.0024 0.002 0.0007 25# 1.48 0.019 0.012 0.0060
0.005 0.0020 26# 1.49 0.022 0.004 0.0060 0.006 0.0038 27# 1.58
0.024 0.006 0.0072 0.006 0.0051 28# 1.56 0.023 0.006 0.0072 0.003
0.0058 29# 1.52 0.025 0.008 0.0070 0.007 0.0043 30# 1.58 0.032
0.006 0.0080 0.006 0.0063 31# 1.57 0.030 0.017 0.010 0.002 0.0072
32# 1.58 0.021 0.005 0.0092 0.006 0.0083
[0073] In the prepared steels, 1-13# are comparison examples, most
of the compositions thereof are within a specified range of T23,
and the contents of B and C do not satisfy a relationship of (%
B)>(% B)>-1.2.times.(% C).sup.2+0.30>(% C)-0.01. It can be
seen from Table 2 that 1-34 are cracked in the post-weld heat
treatment test of a pipe joint; 4-6# are sensitive to reheat
cracking within a wide heat treatment temperature range based on
the evaluation of the implant test; 7-9# are sensitive to reheat
cracking within a wide heat treatment temperature range based on
the evaluation of the implant test and the simulated CGHAZ
isothermal slow strain rate tensile test ; 10# is cracked in the
post-weld heat treatment test of a tube joint, and is sensitive to
reheat cracking at 550 to 750.degree. C. based on the evaluation of
the simulated CGHAZ isothermal slow strain rate tensile test;
11-13# are all sensitive to reheat cracking within a particular
heat treatment temperature range based on the evaluation of the
simulated CGHAZ isothermal slow strain rate tensile test Based on
the evaluation of the above three test methods, these steels have a
very high reheat cracking tendency, reheat cracks are easily
generated in the CGHAZ during the post-weld heat treatment or
exposure at high temperature, and the reheat cracking resistance is
very poor.
[0074] 14-32# are steels designed according to the disclosure and
have compositions satisfying the relationship of (%
B)>-1.2.times.(%C).sup.2+0.30.times.(% C).times.0.01. 14-16# are
not cracked in the actual joint post-weld heat treatment test, and
no cracks are detected on a surface layer and inside the joint by
non-destructive inspection as well. 17-18# are not cracked in the
actual joint post-weld heat treatment test, and no cracks are
detected on the surface layer and inside the pipe joint by the
non-destructive inspection as well; 17-184 are insensitive to
reheat cracking within a wide heat treatment temperature range (500
to 750.degree. C.) based on the evaluation of the implant test.
19-20# are insensitive to reheat cracks within the temperature
range of 500 to 750.degree. C. based on the implant test. 21-23#
are insensitive to reheat cracking within a temperature range of
500-750.degree. C. based on the simulated CGHAZ isothermal slow
strain rate tensile test. Therefore, it indicates that the steels
with compositions satisfying the disclosure are all insensitive to
reheat cracking, no reheat cracks are generated in the CGHAZ during
the post-weld heat treatment or exposure at high temperature, and
the reheat cracking resistance is excellent.
[0075] Test materials comprise bars and pipes with different
specifications. The reheat cracking resistance is very good as long
as the compositions of the steel satisfy the relationship of (%
B)>-1.2.times.(% C).sup.2+0.30.times.(% C)-0.01. Otherwise, the
reheat cracking resistance of the test materials is very poor.
Therefore, it indicates that the reheat cracking resistance of the
test materials is not affected by a manner in which the materials
are formed.
TABLE-US-00002 TABLE 2 Reheat cracking evaluation test results of
steels in examples Determined as cracked Steel Test method Test
result or not Comparison 1# Pipe joint post-weld heat Cracked in
the post-weld heat Cracked examples 2# treatment test treatment
process 3# 4# Implant test Sensitive at 600-750.degree. C. Cracked
5# Sensitive at 650-750.degree. C. 6# Sensitive at 650-750.degree.
C. 7# Implant test Sensitive at 700-750.degree. C. Cracked
Simulated coarse grain zone Sensitive at 650-770.degree. C.
short-time creep fracture test 8# Implant test Sensitive at
600-750.degree. C. Cracked Simulated coarse grain zone Sensitive at
550-750.degree. C. short-time creep fracture test 9# Implant test
Sensitive at 600-750.degree. C. Cracked Simulated coarse grain zone
Sensitive at 550-750.degree. C. short-time creep fracture test 10#
Pipe joint post-weld heat Cracked in the post-weld heat Cracked
treatment test treatment process Simulated coarse grain zone
Sensitive at 550-750.degree. C. short-time creep fracture test 11#
Simulated coarse grain zone Sensitive at 700-750.degree. C. Cracked
12# short-time creep fracture Sensitive at 550-750.degree. C. 13#
test Sensitive at 600-750.degree. C. Examples 14# Pipe joint
post-weld heat Not cracked in the post-weld Not cracked of the 15#
treatment test heat treatment process disclosure 16# 17# Pipe joint
post-weld heat Not cracked in the post-weld Not cracked 18#
treatment test and implant heat treatment process and test both
insensitive at 500-750.degree. C. 19# Implant test Both insensitive
at 500-750.degree. C. Not cracked 20# 21# Simulated coarse grain
zone All insensitive at 500-750.degree. C. Not cracked 22#
short-time creep fracture 23# test 24# 25# 26# 27# 28# 29# 30# 31#
32#
[0076] It will be obvious to those skilled in the art that changes
and modifications may he made, and therefore, the aim in the
appended claims is to cover all such changes and modifications.
* * * * *