LADLE TRANSFER CAR REPAIRED WITHIN SCHEDULED MAINTENANCE SHUTDOWN

Artist's rendering of ladle transfer car with repairs.

Fixing cracks in a hot metal ladle transfer car is not an easy feat under the best of circumstances. Fixing them within a 13.5 hour scheduled maintenance shutdown is truly remarkable. Yet this is exactly what the repair designed by Eichleay Engineers, Inc. (EEI) permitted, thanks to the Algor Finite Element Analysis (FEA) System. The repair was crucial because every eight hours that the ladle transfer car was out of service cost EEI's client, Wheeling-Pittsburgh Steel Corporation (WP), about $185,000.

EEI design engineering capabilities include both process and construction engineering in all major disciplines. EEI's Pittsburgh, Chicago, and San Francisco offices offer clients a wide range of engineering services, from feasibility studies through conceptual design to construction engineering. In conjunction with sister construction companies, EEI offers clients complete design/construct services. Eichleay has been in Business since 1875.

EEI used the Algor FEA System to analyze the stresses that were causing cracks in the ladle transfer car. Modeling of the transfer car was done on a 10 MHz IBM AT compatible with Hercules monitor. Because of the size of the model, analysis was done on a VAX.

The Steel Making Process

The steel making process starts with the smelting of iron ore in a blast furnace. The 2600^oF molten iron, usually referred to as "hot metal", is next transferred to a Basic Oxygen Furnace (BOF). In the BOF high quality steel scrap is added and the metal is "blown" with high purity oxygen, under pressure, to oxidize the excess carbon to carbon dioxide and, simultaneously, bring the metal temperature to about 2900^oF. When the carbon level and temperature are at specified levels, the BOF vessel is tilted, pouring the molten metal into a refractory-lined, preheated ladle.

The hot metal ladle is transferred via overhead crane to the hot metal transfer car. The combined weight of the ladle and hot metal is about 400 tons. The transfer car then transports the metal to the continuous caster facility where the metal is carefully and continuously poured to form 30,000 pound slabs. After solidification, the slabs are further processed into plate and/or sheet product. The caster cannot function without the services of the hot metal transfer car which the only unit in this facility. Thus, the continuous and reliable availability of this transfer car is critical to the production demands of WP.

Norman L. Keller of Eichleay Engineers Inc. (EEI)

Background

In April 1987, EEI inspected a hot metal ladle transfer car at WP's Steubenville South Works facility. Cracks were observed in the bottom inside corners of the car at the point where the main load supporting member reached minimum section. One crack was propagating up the web of the member and the other was propagating along the flange at a weld that joined the flange plates together. The cracking occurred in two corners.

Hand calculations were performed on the car to check the stress levels in the sections where the cracks occurred. The calculated stress intensity S (where S = S max - S min) = 20,750 PSI. With impact due to loading, it was felt that only a few number of maximum cycles were required for failure.

A patch was designed to reinforce the region of the cracks. It was calculated that the patch would reduce stress levels to approximately 60% of the originally calculated 20,750 PSI.

During the repair it was found that the butt welds that tied the two flange plates together were defective (not full penetration welds). At this point it was believed possible that the failure was due to these defective welds. WP opted to modify the repair procedure to reduce the amount of time the car was out-of-service.

In early December, EEI inspected new cracks in the car. The cracks were in the repaired butt welds and also in the fillet welds where the web plates butted into the corners. There was also tearing where the top flange plate butted into the corner. There were no cracks evident in the one-inch patch plate that was put in to reinforce the area where the original cracking was detected. At this time, EEI decided to check the entire car stress situation using FEA.

Analysis

In all, eight ladle transfer car models were created and processed. Four were trial-and-error models to establish a minimum thickness of a repair plate once a repair design was established. Only two of the other four models will be discusses here; SSCAR11, a model of the car as it was originally designed, and SSCAR23, a model of EEI's repair design.

Norman Keller created SSCAR11 using PLTGEN, one of Algor's model building modules. PLTGEN is used to create 3-D models of plates or shells. He started by creating a model of the main box section of the transfer car. It was assumed that all welds were full strength. Loading was 100 tons. Total loading was 400 tons, but due to symmetry only ?of the car was modeled.

Using PLTGEN, Keller entered close to 116 keynodes. Keynodes are the points in space that accurately define the outline of the model. Once the outline was drawn, he defined 133 key regions which are quadrilateral or triangular planes of the model. PLTGEN then further subdivides the key regions based on parameters in the file.

He followed basically the same procedure for the second smaller section of the transfer car. The model of the second section contained close to 24 key nodes and approximately 20 key regions.

When the models of the two sections were complete, they were joined using the Substruct module. The resulting SSCAR11 model had 862 nodes. Keller then used Substruct to add repairs onto SSCAR11 to make SSCAR23. The repairs consisted of:

• extending the corner stiffness plate into the butting box section, and
• inserting an approximately 36" x 36" x 4" thick bearing plate under the four corners of the car so that the loads were more uniformly distributed and the peak stresses were reduced.

SSCAR23 incorporates both plate elements and beam elements. The complete model consisted of 933 nodes. Beam elements were used to get the centroid off-set necessary between the plates of the transfer car model and those in the repair plate model.

The finished models were processed using the static processor for static analysis, SSAP0.

Analysis Results

Inspection of the Algor FEA System-generated tabulation table for SSCAR11 showed that the maximum stress was 47,900 PSI. This occurred at a 5/8 inch thick internal rib plate that is located directly over the truck trunnions. A quick inspection of the tabulation and stress plots showed that a great portion of the car was over-stressed.

To verify that the model was accurately constructed, the original hand calculated stresses from April 1987 were compared with the Algor calculated stresses at the same point. The April 1987 hand calculated stress at the point of failure was S max = 20,750 PSI. The Algor calculated stress at the same point was S max = 21,000 PSI. This was verification that the model and loads were correctly configured.

Inspection of the Algor generated stress tabulations and plots for SSCAR23, showed that there was a marked decrease in stress levels. The table below shows the stress comparison between the original design, SSCAR11, and the repaired car, SSCAR23. In no place do the stresses in model SSCAR23 exceed the 18,000 PSI allowable stress that EEI had established.

STRESS COMPARISON OF SSCAR11 AND SSCAR23
Element Number	Location	SSCAR11 S1-S2	SSCAR23 S1-S2	Percent Reduction
652	Internal Rib	47,900 PSI	12,160 PSI	75%
61	Butt Plate	25,000 PSI	17,150 PSI	31%
501	Bottom Plate	40,300 PSI	4,320 PSI	89%
181	Inside Web	21,000 PSI	10,500 PSI	50%

Conclusions

FEA of the ladle transfer car allowed EEI to quickly determine that the car as originally designed was overstressed. The highly stressed condition occurred in the corners of the car; areas where bending stresses were low but shear and membrane stresses were at a maximum. Without reinforcing the corners, the welds would continue to fail. Weld repairs and maintenance would be a chronic problem.

The repair as shown in model SSCAR23, which consists of localized stress reduction due to the addition of patch plates and the addition of a four-inch thick plate under the entire corner to distribute the load evenly, reduced stresses to an acceptable level.

With the aid of FEA, EEI was able to verify that the repair design was, indeed, a good one. FEA results also convinced WP to go ahead with the repair of the ladle transfer car avoiding chronic repair and maintenance problems. Even more importantly the ladle car did not have to be taken out-of-service for an extended period of time - a very important factor given the cost of an out-of-service car.