Is there a need for post-cutting treatments in shipbuilding?, Presented at the International Conference on Structural Design and Fabrication in Shipbuilding, The Welding Institute, London, 1975

May 1975.

IS THERE A NEED FOR POST-CUTTING TREATMENTS IN SHIPBUILDING?

by

prof.ir. J.J.W. Nibbering

Paper to be presented at the Internatonai Conference on Structural Design and Fabrication in Shipbuilding The Welding Institute, London - 28-20 November 1975.

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The Netherlands. Report No»

IS THERE A NEED FOR POST-CUTTING TREATMENTS IN SHIPBUILDING? by J.J.W. Nibheiing

Post-cutting treatments are applied to free edges for various regions.. Of these smoothing the edges for better appearance or for painting will he

less appealing to the strudtural engineer than the reiioval of roughness, hardened layers and residual stresses from a strength point of view. The outcome of much research (1), (2), (3), (Li) in recent years mainly by Ruge, Schirnmllet, Goldberg and a Dutch Working Group presided by Thomas ('-i)

is that flame-cut edges may have excellent strength properties. There are of course exceptions especially when hand-cutting is considered. But the present paper will mainly be devoted to machine-cutting of straight edges, which will not -be welded in a later stage (slabs). The mentioned Tnachines produce very smooth edges with great accuracy at high speed. Yet, the

latter has given some concern because a high speed involves fast cooling -and consequently hardening.

Fortunately the thickness of the hardened zone is smaller the higher the cutting speed. Furthermore, contrary to what might -be expected the residu-al stresss are rather heneficient than hamfül, hein-g mostly compressive

There are two aspects to be considered when discussing the strength of un-treated edges: the behaviour at low temperature and the fatigue-strength. Fear for brittle fracture has sometimes led to the requirement that a higher grade of steel should be applied when the flame-cut edges remain untreated. It will be seen that there is not any reason for this.

Inconnectiontofttiguei4has_been_found_th at only more or les s in cident -al phenomena like irregularities due to losing a cu-t or disturbances of the gasfiow may cause trouble. The fatigue-strength in presence of such defects may become as low as half of the value in absence of defects, (1LiO N/mm2 instead of 260/mm2). The faults are normally very few in number and öan certainly be detected by an- experienced eye. Then it is illogical to grind or mill say 10 meters of an edge in order to eliminate one or two defective points. Apart from this, these points may often very well remain as they are. This is demonstrated -in figure 1. It shows two faults at -the flame-cut edge of the flange of a beam ó a full-scale specimen. During fatigue-load-ing a crack developed at both faults; but practically at the same time cracks developed at the toes o-f two fillet-welds on top of the flange,

Professor State -Uhiversity c-hent; Reader Delft University of Technology.

(positions O and io). The same was observed in one other specimen (out of a total of 7).

The point in connection to this is that there is little logic in improving a flame-cut edge, when other structural defects, like undercuts, are toler-ated. It is like the chain with a number of weak links.

In connection to figure 1 it is worth emphasizing that generally people are too afraid of, cracks. In redundant structures like ships cracks in one or two longitudinals do not endanger a ship. The loads are simply shifted to othei members. Thereabove the critical crack length for unstable fracturing in current ships' steels is so large that it takes far more than one survey period to reach it. 'Indeed cracks should be looked upon in a realistic way, in conformity with the principle of !!fail_saf&1 design for ships.

It will be clear that local grinding or milling is recommended for the eliminatiOn of defective spots. But weld-repairing of defects, - eventually

after removing some material - , should never be done., except in case of really large faults, (say deeper than 5 mm). In most cases it will not

re-store strength, because at the ends of the repair weld some discontinuity is unavoidable. Grinding it away is exactly the same as grinding away the

original cut-defect, so why welding? Sabelstrm (5) in an unpublished re-port mentions fatigue-strengths of only 150 N/mm2 for weld-repaired defects

(2 X 106 cycles).

Local grinding or milling has an additional important advantage The author has observed several timesthat -w-i-t-hrout4ne-grinding.. of all flame-cut edges some defects which really needed removal, remained present! Only the depth was reduced by the amount of general grinding.

2. The strength of flame--cut edges at sub-zero temperatures.

Most of the research connected to flame-cUt edges has been concentrated on the influence of cutting on the fatigue-properties.

Yet there exists probably an enormous amount of unreported results of ex periments with large specimens containing flame-cut edges. Such. data might particularly be collected in laboratoiies disposing of large testing

facil-ities.

-The. Delft Ship Structures Laboratory is one of these. It has a 1000 tons low-cycle fatiguetesting machine for specimens up to 13 m X 5 m.

In the course of years numerous large welded structures (of the type of

fig. _{ii) and long plate specimens have been subjected to static or cyclic}

loading at temperatures down to -110°C. The total length of flame-cut edges in all specimens has been at least 500 m. All specimens fractured at weld-details and -defects, or artificially made notches, or

fatigue-cracks. Never has a brittle fracture started at a flame-cut edge of a specimen, despite the fact that deformations well in the plastic range have occurred Even in the cases that severe shock-loading was applied, the flame-cut edges behaved well.

It should be added that several kinds of shipbuilding steels had been

used: Mild steel grades A and D in semi-killed and killed condition (St. 12), C-Mn St. 52 and Nb-containing St. 52. The thickness range was from 13 to

6 mm.

This experience virtually excludes he possibility that a brittle fracture initiates at a flame-cut edge.

Support to this opinion comes from Goldberg (2) arid from special investiga--tions carried out in Delft in connection to the problem (q), (6).

Figures 2 and 3 show the behaviour of two specimens, which after appreciable overall plastic straining fractured. The fracture of fig, 2 was called semi-brittle. It is not of significance fcr ships because it happened after de-formations which never will occur in practice.

For those who are familiar with C.0.D.-testing this will be immediately oh-vious. Indeed, in most of the C.0.D.-tests similar brittle fractures occur. What counts s, whether the fractures develop aftet- a satisfactory amount of local deformation (crack-opening-displacement) or not. Fror this it mày be understood that Goldberg's results were satisfactory They were even

very satisfactory because most of the spedimens fractured by shearing i-stead of semi-brittle.

Figures , S and 6 from (), (6) give evidence of the fact that Up to 3%

prestressing of flame--cut specimens at -10°C and -30°C has little or no influen ce on the fatigue-strength.

From this two conclusions may be drawn in connection to brittle fracture: None of the tested bars fractured during prestraining at low temper-ature not even the St 2 grade A.

In not any of the hardened flame-cut edges (predominantly riartensite) cracks had developed as a consequence of the straining.

This was concluded from careful observations with a magnifring glass and particularly from the fact that the prestraining had little or no effect

on the fatigue-behaviour.

Where some loss of fatigue-strength was found (St.. 52, fig. 4) it was not due to microcracking but to, the partial elimination of the beneficient com-pressive residual stresses at the edges.

In ,( i.) the result of later observations with a microscope is reported. Specimens cut with a speed of 250 mm/min.. could withstand 1,5% tothi strain before tiny cracks developed in the 0,02 mm thick high-carbon martensite

layer. It is very unlikely from a fracture-mechanics point of view that cracks of that length can develop into brittle cracks n the underlying o,S mm deep low-martensite zone.

3. _{The fatigue-strength of flame-cut edges.}

The main points in connection to fatigueloading have already been stated in section 1. 0f all parameters of interest in connection to the quality of

flame-cut edges the presence of faults or defects is most proicinent. Hardness, residual stresses, roughness, depth and microstructure of hardened zone are orilyof interest When faults are not present.

From Goldberg?s data in (1) the worst result 'for a defective specimen may be estimated to be 200 N/mm2, (N 2. x 106).. This is appreciably lower than his values for normal quality cuts (280 N/mm2) and low quality ones (240 N/mm2). The steel was St. 52 - 3 (0,18 C; 1,4 Mn, 0,025 Nb), (being equivalent to ASTI'I A 572, Grade 55). Plate thickness was 30 mm.

The results given in (4) are slightly lower for cuts without defects, but droptol40N-/mm2for- a-speciL'men with. an- accidentalfaul.t. .CSt Fe..5.10T

C 3, equal to Euronorm 25-67; 0,20 C; 1,30 Mn, 0,37 Si).

Some unpublished new results of the Dutch Working Croup 1.913 are given in table 1.

In (14), (6) a clear relation has been found between the fatigue-limit and the magnitude and sign of the residual stresses as shown in fig. 7. It ex-plains why for steels in which compressive residual stresses are present -. due to martensite forming - , removal of the martensite layer mostly

results in a lower fatigue-lir!iit. Prestraining. has more or less the same effect.

Figure 7 also explains the very low fatigue-limit of the Fe-E-42 wIth high yield point which had low residual edge stresses. It should once again be emphasized that the mentioned high fatigue-limits (up to 300 N/mm2) found

Table 1.

Material Thickness

atiguè strength 2 x cycles repeated loading Loading Observations -without defets wi+h defects 2% strained defects removed by grinding

FeE 2 O 25 190 150 175 defect made by 2 sec. stop

L

Fe 52 360 25 27b 160 230

[

M M M M

110

J

devia-tion of straight line

Fe 52 360 35 325

220 bending

fine grain coarse grain

for most defect-free edges in St. 52 are not of interest for practice. As stated in the introduction the most realistic values are probably those for untreated edges with delects.. It is thought that a stop of the cutting-machine for 2 seconds produces defects representative for practice.

The corresponding fatigue-limit is lL0-l50 N/mm2 (for St:. 52 and St. '42).

For those cases that defects are removed by local grinding or milling the fatiguestrength is equal to that for machined edges. It ranges from ₁₇₅

I'I/mm2 to

230

L _{Final observations.}

The scope of this conference and the need for short papers made it undesir-able to discuss thoroughly the. many interesting aspects of flame-cutting. The interested reader is referred to the cited literature for more informa-tion. about:

- Influence of cutting speed - metallurgy - hardness - measuring of rough-ness and esiduai stresses - influence of thermal stress relief - and pos-sibilities and effects of preheating.

References.

i GOLDBERG, F.. 'Influence of thermal cutting and its quality on the fatigue-strength of steel'. N.J. , Sept.

1973.

GOLDBERG, F. 'The influence of thermal cutting on the static strength of steels'. I.I.W./I.I.S. Doc.

_I-48L-72

in brenngeschnttenen Stahlblechen aus St. E 36'. Mittei-lungen der BEFA, Nr. 12, 197 1.

'4 THOMAS, H., DE BACK, J., MULLER, T., NIBBERING, J., VERWEY, C..

yaNK, R. , BOS, T. 'The properties of flame-cut edges'. Report of W..G. 19130f the Dutch Inst. of Welding, May 1973.

5 SABELSTROM, W. Report thd. 27-li-1970 of the Welding Dept. of the Royal Inst. of Technology - Stockholm.

6 NJBBERII'JG, J. , VONK, R. 'Residual stresses at flame-cut

7

SCHOLTE, H. 'Fatigue experiments with full-scale

bottom-longitudinals of St. '42 and St. 52 under alternating bending'. Report No. Ï95 S.S.L. Delft, March 1975.

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