Spot welders take short, sharp gulps of power and this is no exception. It is rated at 70kVA which corresponds to a peak line current of 100A. The average current will be much lower, so the built-in D32 (32A, D-curve) MCB suffices. Like any 32A MCB this will react to a prolonged steady current exceeding 32A by a certain factor, but different tripping curves allow different maximum peak currents before their instantaneous magnetic trip mechanism reacts. Most wiring uses B-curve or for more demanding loads C-curve MCBs, with D-curve being reserved for very high-inrush and surge loads such as this. It is designed to ride out brief pulses of up to 10xIn = 320A, which allows for the momentary 70kVA demand of the welder.
Since the manufacturers will have chosen the MCB with knowledge of the demands of the unit, that is probably the minimum rating that will allow it to be used to its full capability without nuisance tripping, so you would need to provide equal or greater rating upstream. The challenge is that the low sensitivity of a D-curve MCB to short-circuit currents puts more onerous requirements on the loop impedance of the wiring to ensure faults are cleared rapidly. Sometimes that is easy to satisfy, sometimes not.
Because of the unusual nature of the load, the suitability of the supply should be checked, preferably by someone familiar with industrial equipment as this is rather out of the sphere of many electricians. Spot-welders and their like can be so disruptive to supplies that in an industrial setting, it is sometimes necessary to advise the DNO that such a load is to be connected and they are at liberty to limit the maximum rating to avoid other customers' lights pulsating in response to the voltage fluctuations it creates.
